Fungi - Plant Pathogenic

Fungi: Plant Pathogenic
A B Gould, Rutgers University, New Brunswick, NJ, USA

ª 2009 Elsevier Inc. All rights reserved.
Defining Statement Pathogen Groups

Introduction Diagnosis
Fungal Characteristics Control
Symptoms of Plant Disease Caused by Fungi Further Reading
Glossary hyperplasia Excessive cell division.

anamorph The sexual reproductive state of a fungus. hypertrophy Excessive cell enlargement.
anastomosis Hyphae of two fungal isolates (both from hypha Microscopic, tubular thread; structural unit of a
the same anastomosis group) fuse and undergo fungus.
plasmogamy to develop a network. mycelium Collective group of hyphae; makes up the
biotroph An organism that derives nutrition only from thallus of a fungus.
living sources of organic matter (obligate parasite). necrosis Killing of cells by fungal infection.
chlamydospore Asexual resting spore formed as plasmodium Multinucleate mass of protoplasm lacking
hyphae cells develop a thick wall and separate. a cell wall.
conidium Nonmotile, asexual spore produced at the tip rhizomorph Thick strand of somatic hyphae that
or side of a supporting structure known as a resembles a root; serves a survival and dispersal
conidiophore. purpose for some basidiomycetes.
damping-off Seeds and seedlings are killed by soil saprophyte An organism that derives nutrition from
organisms before (preemergent) or after (postemergent) nonliving sources of organic matter.
they break the soil surface. sclerotia Compact masses of mycelium consisting of a
dikaryon Cell with two genetically different but central core of hyphae protected by a thick-walled rind;
compatible haploid nuclei (a characteristic of fungi in the survival structure.
Basidiomycota). sporocarp Fruiting structure-bearing spores.
etiolation Excessive elongation. teleomorph The sexual reproductive state of a fungus.
holocarpic Entire thallus converts to a fruiting thallus Vegetative (nonreproductive) body of a fungus.
structure. zoospore Motile fungal cell with one or more flagella;
holomorph Teleomorph (sexual) and anamorph used for dispersal.
(asexual) reproductive states combined.
Abbreviations
ELISA enzyme-linked immunosorbent assay RAPD random amplified polymorphic DNA
LSD lysergic acid diethylamide RFLP restriction fragment length polymorphism
PARP pimaricin þ ampicillin þ rifampicin
þ pentachloronitrobenzene
Defining Statement Introduction
An overview of the phyla of plant pathogenic fungi is The ‘fungi’ encompass microorganisms that fill a similar
presented, summarizing hyphal characteristics, reproduc- ecological niche and yet are distributed among several
tion, dispersal, and survival mechanisms, symptoms, taxonomic groups, broadly classified as true fungi and
fungal diagnostics and control, and definition of com- pseudo-fungi (fungus-like). As such, the fungi are exam-
monly used terms. For each pathogen group, examples ples of convergent evolution, where organisms that are
of common plant pathogenic fungi and the diseases they not related evolve similar traits to exploit a similar envir-
cause are provided. onment. In their primary role, fungi are extremely
457
458 Fungi | Fungi: Plant Pathogenic
important as agents of decay. Many fungi are symbionts; (with crosswalls) or aseptate (coenocytic, without cross-
some are parasites and use other organisms (including walls). Septa usually contain small pores to ensure
plants and animals) as a source of food; others are partners continuity with other cells. Fungal hyphae elongate by
with algae to form lichens; some partner with plant roots apical growth (from the tip). Hyphae grow over a sur-
to form mycorrhizae; and others are endophytes, living face stratum, may penetrate it, or may produce an
within plant tissue without discernible changes in host aerial mycelium. In culture, fungi form colonies,
development. Some fungi are capable of existing in more which appear as collections of hyphae with or without
than one mode. spores that arise from a central cell or grouping of cells.
Although the formal study of fungi, called mycology Fungal cells from different hyphal strands may often
(Greek for Mycos (fungus) þ -logy (study)), is about 250 anastomose, or fuse, to form a three-dimensional net-
years old, these organisms have had an impact on human work. This process permits the development of
social and economic development throughout history. specialized survival or dispersal structures such as rhi-
Fungi are a source of food, are used to produce food zomorphs, sclerotia, and fruiting structures, also known
(cheese, leavened bread, and wine), and can also destroy as sporocarps.
food at any stage of production, processing, or storage. Hyphal cells are bound by a cell envelope called the
Many fungi are poisonous; some produce hallucinogens plasmalemma (or plasma membrane) that contains the
or toxins and others produce enzymes that degrade fab- sterol ergosterol. The plasmalemma of fungi differs
rics, leather goods, and wood products. Societies that from that of plants, which contain a phytosterol, and
depended on a single crop for major sustenance have animals, which contain cholesterol. Outside the plasma-
been devastated by fungal disease. For example, the lemma is the glycocalyx, which appears as a firm cell
potato crop, a major source of food for Irish peasants in wall (for most fungi) or as a slimy sheath (as in the
the 1800s, was destroyed by a fungal disease, called late slime molds). The glycocalyx is composed chiefly of
blight of potato, in 1845–46. The impact of this disease polysaccharides that differ among fungal groups
was enormous, resulting in massive starvation and emi-
(Table 1). Primary cell wall polysaccharides of the
gration of the Irish people for years to come. The
Ascomycota, Basidiomycota, and Deuteromycota
discipline of plant pathology had its origins in the scien-
(mitosporic fungi) include chitin ( -(1!4) linkages of
tific and political controversy caused by this disease.
N-acetylglucosamine) and glucans (long chains of glu-
Other fungal diseases that have resulted in severe crop
cosyl residues); in the Zygomycota, chitosan, chitin, and
loss throughout recorded history include the cereal rusts
polyglucuronic acid are present; and the Oomycota
and smuts, ergot of rye and wheat, brown spot of rice,
contain cellulose ( -(1!4) linkages of glucose) and
coffee rust, Sigatoka disease of banana, chestnut blight,
glucans. Cell walls also contain proteins and, in some
and the downy and powdery mildews of grape. Those
fungi found consistently in association with a particular fungi, dark pigments called melanins. Strong fibers
plant disease are called pathogens. called microfibrils, which lend support to the fungal
cell wall, are composed of chitin, glucans, and cellulose.
Fungal cells may be uni- or multinucleate. Nuclei are
haploid (most often) or diploid. Cells with genetically
Fungal Characteristics identical haploid nuclei are monokaryotic; cells with two
genetically different but compatible haploid nuclei
Fungi are eukaryotic, heterotrophic (lacking photosynth- are dikaryotic (a characteristic of fungi in the
esis) organisms that, in most fungal groups, develop a Basidiomycota). Compared to animals and plants, the
microscopic, tubular thread called a hypha (pl. hyphae). A nuclei of fungi are relatively small with fewer chromo-
group of hyphae is known collectively as a mycelium (pl. somes or less number of DNA base pairs. Plasmids
mycelia), which makes up the vegetative (nonreproductive) (extrachromosomal pieces of DNA that are capable of
body or thallus of the fungus. In some fungi, the thallus is independent replication) are found in some fungi, includ-
single-celled (as in the yeasts) or may be plasmodial (with- ing the common yeast fungus, Saccharomyces cerevisiae.
out a cell wall, as in slime molds). Although fungi lack a Other organelles found in fungal cells include mitochon-
complex vascular system, they can form specialized struc- dria (energy-producing organelles), which vary in size,
tures for survival, dispersal, and spore production. Most form, and number; vacuoles, which serve to store water,
fungi, except for a few groups, are not motile. nutrients, wastes, or enzymes such as nucleases, phos-
phatases, or proteases; and plastids, which contain
pigments and enzymes and may store food. Fungal cells
Hyphae and Fungal Cells
accumulate reserve carbon materials as glycogen,
Fungal hyphae differ in diameter among species (3–4 to lipids, or low molecular weight carbohydrates such as
30 mm or more micrometer wide) and may be septate trehalose.
Table 1 Classification and characteristics of plant pathogenic fungi
Kingdom
Protozoa Stramenopila Eumycota
Plasmodiophoromycota Oomycota Chytridiomycota Zygomycota Ascomycota Basidiomycota
Habitat Aquatic Aquatic Aquatic/ Terrestrial Terrestrial Terrestrial

terrestrial
Form of thallus Plasmodium Coenocytic hyphae Globose or ovoid Coenocytic Septate hyphae; Septate hyphae
thallus (no true hyphae; some are
mycelium) rhizoids, single-celled
stolons (yeasts)
Ploidy Haploid Diploid Diploid Haploid Haploid Dikaryotic, haploid
Chief component(s) None Cellulose/glucans Chitin Chitin/chitosan Chitin/glucans Chitin/glucans
of the cell wall
Motile stage Zoospores with two Zoospores with two Zoospores with None None None
anterior, unequal, flagella: one anterior one posterior
whiplash flagella tinsel and one posterior whiplash
whiplash flagella flagellum
Food relationship Biotroph Facultative, biotroph Biotroph Facultative Facultative, Facultative, biotroph
biotroph
Asexual Zoospores in Zoospores in sporangia; Holocarpic Sporangiospores Conidia on Conidia, chlamydospores, oidia
reproduction zoosporangia chlamydospores in sporangia conidiophores
(singly or in
fruiting
bodies)
Sexual Resting spores, the result Oospores, the result of Not confirmed Fusion of Formation of Formation of four basidiospores on a
reproduction of fusion of zoospores fusion between male isogametes to ascospores basidium; fusion of pycniospores and
to form a zygote antheridia and female produce a within an receptive hyphae (rusts); fusion of
oogonia zygospore ascus compatible mycelia (smuts)
Reproduced with permission from Gould AB (2008) Plant pathogenic fungi and fungal-like organisms. In: Trigiano RJ, Windham MT, and Windham AS (eds.) Plant Pathology Concepts and Laboratory
Exercises, 2nd edn. Boca Raton: CRC Press.
Nutrition host. Biotrophs, which are highly host specific, do not

readily kill their food source, thus ensuring a steady
Fungi lack chlorophyll and, unlike plants, cannot manu-
supply of nutrients. Some biotrophs produce haustoria,
facture their own food. Fungal growth requires an external
which are structures designed for penetration and absorp-
food source, water, and appropriate environmental condi-
tion of nutrients. Although a haustorium may penetrate
tions. Growth ceases when any of these become limiting.
the host cell wall, it does not penetrate the host plasma-
Nutrients required by fungi include a source of carbon
lemma, thus essentially remaining outside the host cell
(sugars, polysaccharides, lipids, amino acids, and proteins);
nitrogen (nitrate, ammonia, amino acids, polypeptides, and while nutrients are transferred into the fungus. Compared
proteins); magnesium, phosphorus, potassium, and sulfur; to strict saprophytes, biotrophs are relatively rare. Classic
and trace elements such as calcium, copper, iron, manga- examples of pathogenic biotrophs include the powdery
nese, molybdenum, and zinc. mildew fungi and rusts.
Fungi obtain nutrients by using an absorptive mechan- Fungi may also be classified as facultative saprophytes
ism: hyphae secrete digestive exoenzymes into an or facultative parasites; these organisms are more versatile
external food source, and nutrients are carried back and utilize both living (as parasites) and nonliving (as
through the fungal cell wall and stored in the cell as saprophytes) sources of carbon. Facultative saprophytes
glycogen. This process requires the presence of free live primarily as parasites; they attack living hosts but
water. The source of carbon used by a given fungus is then subsist between growing seasons as saprophytes
limited only by the exoenzymes the fungus produces within the normal soil microflora. Alternatively, faculta-
(Table 2). tive parasites under normal conditions utilize nonliving
sources of carbon (as saprophytes); given fortuitous cir-
cumstances, however, these organisms will attack highly
Food relationships susceptible living plant tissue, as parasites.
Most fungi are scavengers or decay organisms that use Other pathogens, called necrotrophs, survive as sapro-
nonliving sources of organic material as a source of food. phytes in the absence of a living host, but given the
These organisms are called saprophytes, and along with opportunity, will kill a host plant and subsequently feed
bacteria, recycle carbon, nitrogen, and mineral nutrients. on the dead plant tissues. Necrotrophs produce secondary
At the other end of the spectrum are biotrophs (some- metabolites that are toxic to susceptible host cells; cells
times called obligate parasites) that derive all nutrients killed by these toxins are degraded by fungal enzymes,
necessary for growth or reproduction only from a living and the cell constituents are used as food. Tissues killed
Table 2 Function of digestive exoenzymes produced by pathogenic fungi
Enzyme Substrate Function
Amylase Starch (storage polysaccharide in plant cells) Hydrolyzes starch to glucose, which is used as
food by pathogen
Cellulase (cellulase C1, Cellulose (insoluble, linear polymer of -(1!4) linkages Softens cell walls, facilitating spread
C2, Cx, and - of glucose); skeletal component of primary and
glucosidase) secondary plant cell walls
Cutinase Cutin (long-chain polymer of C16 and C18 hydroxy fatty Facilitates direct penetration into host cuticle;
acids); with waxes and cellulose, main component of enzyme necessary for pathogenicity
cuticle
Hemicellulase (e.g., Hemicellulose (mixture of amorphous polysaccharides); Role of these enzymes in pathogenesis is unclear
xylanase, arabinase) component of primary and secondary cell walls
Ligninase Lignin (complex, high molecular weight polymer made A few basidiomycetes degrade lignin in nature;
of phenylpropanoid subunits); major component of brown rot fungi degrade lignin but cannot use it
secondary cell wall and middle lamella of xylem tissue as food; white rot fungi can do both
Lipase Fatty acid molecules; major components of plant cell Fatty acid molecules used as a source of food by
membranes; also stored in cells and seeds and found pathogens
as wax lipids on epidermal cells
Pectinase (pectin Pectin (chains of galacturonan molecules ( -(1!4)-D- Degrades middle lamella and primary cell wall,
methyl esterase, galacturonic acid) and other sugars); main component macerating host tissue; facilitates penetration
polygalacturonase) of the middle lamella and primary cell wall and colonization of host
Proteinase Protein (major components of enzymes, cell walls, and Disruption of cell membrane and enzymatic
cell membrane); enzymes hydrolyze protein to smaller activity affects host cell function; precise role of
peptide fractions and amino acids these enzymes in pathogenesis is unclear
Reproduced with permission from Gould AB (2008) Plant pathogenic fungi and fungal-like organisms. In: Trigiano RJ, Windham MT, and Windham AS
(eds.) Plant Pathology Concepts and Laboratory Exercises, 2nd edn. Boca Raton: CRC Press.
Fungi | Fungi: Plant Pathogenic 461
by necrotrophs often appear blackened or sunken. An produce special sex cells (gametes or gamete nuclei)
example of a necrotroph is Monilinia fructicola, which that fuse to form a zygote. Fertilization is a two-step
causes brown rot of peaches. process: (1) plasmogamy, where the two nuclei join
Finally, the hemibiotrophs function as both a biotroph together in one cell; and (2) karyogamy, where these
and a necrotroph during the life cycle. At first, these fungi nuclei fuse to form a zygote. In different hyphal groups,
subsist as biotrophs, growing between the plasma mem- karyogamy does not necessarily occur immediately fol-
brane and the cell wall of living cells, and switch later to a lowing plasmogamy. Examples of sexual spores include
necrotrophic phase, killing all the colonized cells and ascospores, basidiospores, oospores, and zygospores.
utilizing the dead tissues for nutrients. Those parasites The thallus of most true fungi is haploid (contains one
found consistently with a given plant disease are set of chromosomes); thus the diploid (two sets of chromo-
called pathogens. The soybean anthracnose pathogen, somes) zygote that results from karyogamy must undergo
Colletotrichum lindemuthianum, is a hemibiotroph. meiosis before a haploid thallus can then develop. In
contrast, fungus-like species in the Oomycota possess a
diploid thallus, and meiosis occurs in the gametangia as
Reproduction
the gametes are formed. Homothallic fungi (producing
Most fungi produce spores, which are small, microscopic both male and female gametangia on a single mycelium)
units consisting of one or more fungal cells. Spores are are self-fertile. These gametangia may be differentiated
produced asexually or sexually and provide a dispersal into male (antheridium) and female (oogonium) structures
and/or survival function for the fungus. The size, shape, or may be undifferentiated. In contrast, heterothallic fungi
color, and genesis of spores are also used as taxonomic (producing male and female gametangia on separate indi-
criteria. viduals, or mating types) are self-sterile. Mating types are
usually designated by using letters (e.g., A and A9) or as
Asexual reproduction plus (þ)/minus ().
Asexual reproduction, the result of mitosis, results in
progeny that are genetically identical to the parent. Anamorph–teleomorph relationships
Spores that result from asexual reproduction include con- Most fungi exhibit both sexual and asexual reproduction.
idia, chlamydospores, zoospores, and sporangiospores. These fungi are pleomorphic, having more than one form
Most true fungi in the Ascomycota, Basidiomycota, or state. The sexual state of a fungus is called the tele-
and those classified as deuteromycetes (mitosporic omorph (in the literature, often referred to as the perfect
fungi) produce conidia (nonmotile, asexual spores) at state), and the asexual state is called the anamorph (the
the tip or side of a supporting structure known as a imperfect state). The organism in its totality (both ana-
conidiophore. Conidiophores are arranged singly or in morph and teleomorph) is the holomorph. The anamorph
fruiting structures such as synnema (stalks of fused con- or teleomorph in some fungi may be lacking or not
idiophores), acervuli (flat, saucer-shaped beds of short described. Those fungi that lack a known teleomorph
conidiophores that grow between host tissues and the are artificially described as mitosporic fungi, imperfect
epidermis or cuticle), and pycnidia (flask-shaped struc- fungi, or deuteromycetes.
tures lined with conidiophores). Many true fungi, particularly in the Ascomycota and
Chlamydospores, found in many groups of fungi, are Basidiomycota, produce sexual and asexual states at dif-
thick-walled resting spores formed as hyphae cells ferent points in the life cycle. As a result, these fungi are
develop a thick wall and separate. Sporangiospores, pro- associated with two names: one that refers to the ana-
duced by fungi in the Zygomycota, are also borne in a morph at the time it was described and the other that
sporangium. Zoospores, produced in the Oomycota, refers to the teleomorph at the time that it was described.
Plasmodiophoromycota, and Chytridiomycota, are motile This can be confusing from a taxonomic standpoint.
spores with one or more flagella. In the Oomycota, zoos- When an anamorph is finally associated with its corre-
pores are borne within a sac-like structure called a sponding teleomorph, the holomorph is referred to by its
sporangium, supported by a stalk called a sporangiophore. teleomorph name. Although the fungus may reside in
Zoospores are specialized for short-distance dispersal, different reproductive states during its life cycle, the
responding to external stimuli, such as root exudates genetics of the two states are the same; thus molecular
(chemotaxis), for identifying suitable penetration sites, biology has been helpful to link anamorphs to their cor-
where they encyst and initiate new infections. responding teleomorphs. Examples of holomorphs
include Claviceps purpurea (teleomorph) ana. Sphacelia sege-
Sexual reproduction tum (anamorph) (causal agent of ergot of rye and wheat);
Sexual reproduction results in genetic recombination, Cochliobolus heterostrophus ana. Cochliobolus heterostrophus
producing offspring that are genetically different from (southern corn leaf blight), and Venturia inaequalis ana.
either parent. In the fertilization process, gametangia Spilocea pomi (apple scab).
In general, the sexual and asexual reproductive strate- distances. Spores are mostly commonly dispersed by
gies exhibited by many pathogenic fungi may confer water (rain splash and flowing water), air currents, insects,
different ecological advantages. When food is abundant or with seed or plant parts. For example, the sporangia
and dispersal is important, fungi reproduce asexually. and zoospores of pythiaceous fungi (oomycetes), which
Conversely, when food is limited or dispersal is not as cause root diseases of many important crops, are easily
critical, fungi reproduce sexually. For example, the fungi dispersed over long distances as water moves through the
that cause powdery mildew are biotrophs and produce soil profile. Urediniospores of the stem rust of wheat
easily dispersed conidia (asexual spores) during the grow- pathogen, Puccinia graminis, are blown hundreds of miles
ing season when host material is abundant. At the end of from winter wheat crops in the south to the spring wheat-
the growing season, sexual reproduction is triggered by producing portions of the United States along a corridor
host and environmental factors. The resulting ascocarp known as the ‘Puccinia pathway’. Insects are intimately
(called a cleistothecium) overwinters in association with associated with the life cycle and spread of many fungi.
plant parts or debris, releasing meiotic ascospores the For example, Ophiostoma ulmi, the pathogen that causes
following growing season. Dutch elm disease, is vectored by bark beetles that spread
fungal spores during feeding. This fungus is also dispersed
from tree to tree through root grafts. Fungi dispersed with
Dispersal and Survival
seed, such as Tilletia caries (stinking smut of wheat) or
Most fungi that cause disease in plants spend a portion of C. purpurea (ergot of wheat and rye), are conveniently
their life cycle as parasites and the remainder as sapro- situated near their food source at the beginning of the
phytes, using nutrients found in soil or in plant debris. infection process. Plant pathogens are most effectively
Hyphal growth ceases during adverse environmental con- dispersed across land masses or vast bodies of water by
ditions or when nutrients are limiting. Pathogenic fungi human activities. For example, both Cryphonectria parasitica
must develop mechanisms to disperse to new hosts or to (chestnut blight) and Phytophthora parasitica (late blight of
otherwise survive until conditions improve. Spores used potato) destroyed vast acreage of host plants when moved
for dispersal include conidia and zoospores; spores and with infected host plant material from the pathogen site of
other structures that serve a survival purpose include origin to an exotic location.
chlamydospores, oospores, sclerotia, teliospores (the rust Active dispersal in pathogenic fungi is limited. As
and smut fungi), and zygospores. Other spores, such as mentioned previously, zoospores produced by several
ascospores and conidia, may survive adverse environmen- fungal groups actively seek new root penetration sites,
tal conditions within fruiting structures such as pycnidia, using stimuli such as chemicals, oxygen, or light for
perithecia, pseudothecia, and cleistothecia. Pathogens direction (taxis). Zoospores do not swim linearly, how-
may also survive and disperse as spores or hyphae in ever, and instead are pushed or pulled by flagellar motion,
association with infected or infested seed and other frequently changing direction when hitting an obstacle in
plant parts. the soil. Zoospores are best for dispersal over short
distances.
Spore liberation and pathogen dispersal Rhizomorphs, common in some basidiomycete fungi,
Fungal survival depends on the exploitation of new food are thick strands of somatic hyphae that resemble roots.
sources; thus, liberation from the parent mycelium and Pathogens such as Armillaria mellea disperse from an exist-
subsequent dispersal are critical processes. Spore libera- ing carbon source, which may be a plant host or woody
tion from the parent mycelium is passive or active. Spores plant debris to new substrates as the active meristem of
such as conidia may be passively liberated through out- the rhizomorph grows through the soil. Rhizomorphs
side forces, mechanically (splashing water, wind, animal have a dark outer rind that protects an inner cortex of
disturbance, or cultivation equipment) or by electrostatic active hyphae. These root-like structures normally pene-
repulsion between the spore and its subtending stalk trate new host roots, usually trees, near the soil line. A
(sporophore). An example of a pathogen commonly lib- new mycelium then proceeds to grow up the trunk just
erated by air currents in greenhouses is Botrytis cinerea. under the bark, killing vascular tissues and causing tree
Active spore liberation occurs due to mechanisms within decline or death.
the fungus itself, such as changes in water pressure within
cells. Ascospores are often forcibly discharged from the Survival
ascus (supporting sac) in many ascomycetes, and changes As mentioned previously, fungi survive adverse environ-
in cell shape in some rust fungi, such as species of Puccinia, mental conditions as spores (e.g., oospores, zygospores,
actively propel aeciospores into the air. chlamydospores) or other specialized structures that act
Dispersal is also achieved by passive (most common) as propagules or sources of inoculum. Common to most of
or active means. Passively dispersed (vectored) pathogens these structures is a thick, protective outer cell wall (as in
may, depending on the mechanism, travel short or long spores) or rind (as in rhizomorphs and sclerotia). Survival
structures may be the result of both sexual and asexual as the common yeast (S. cerevisiae), however, do not need
reproduction, or in certain groups, may germinate to oxygen for respiration and are anaerobic or fermentive.
release sexual (as in C. purpurea) or asexual (as in B. cinerea) There are a few plant pathogens that derive energy by
spores. With the exception of rhizomorphs, which also either oxidation or fermentation; these are called facultative
serve a dispersal purpose, most survival structures are fermentives. Although light is not a requirement for fungal
passively dispersed, if at all. growth, some species produce melanins in the hyphal wall
Oospores and zygospores are the result of sexual to protect against damage from sunlight.
reproduction in the Oomycota and Zygomycota, respec- Fungi must tolerate temperatures that may fluctuate
tively. An oospore forms when an oogonium (female considerably during the day or throughout the year. Most
gamete) is fertilized by an antheridial (male gamete) fungi grow well between 10 and 40 C (with an optimum
nucleus; a characteristically thick wall and food reserves between 25 and 30 C) and are considered mesophilic.
help to ensure survival. Zygospores, which form following Species that grow best at 40 C or higher are thermophilic;
fertilization of gametes that are morphologically indistin- those that grow at temperatures of less than 10 C are
guishable, are protected by a thick wall containing psychrophilic. Fungi function best at a pH of 4–7.
melanin and ornamented by warts. As in the Oomycota,
the thick cell wall structure as well as abundant lipid
reserves facilitates long-term survival of the fungus.
Meiosis does not occur until conditions are sufficient for Symptoms of Plant Disease Caused by
zygospore germination, which results in the production of Fungi
a sporangium with zygospores.
Asexually produced chlamydospores are found in Symptoms are the visual manifestation of the infection
many fungal groups. Hyphal cells, either within the strand process. Symptoms vary depending on the host, the
(intercalary) or at the terminus, accumulate a thick wall infected plant part, and the environment. Symptoms
and nutrient reserves, round up, and separate. Although caused by fungal pathogens can be similar to those caused
chlamydospores are common to soilborne fungi, they are by other biotic disease agents, such as bacteria and viruses,
associated with aerial pathogens as well, such as as well as abiotic (environmental) disease agents, includ-
Phytophthora ramorum, the causal agent of sudden oak ing extremes in moisture and temperature. Symptoms
death and ramorum blight. caused by fungal pathogens are broadly described as
A common survival, or resting, structure is the scler- abnormal growth, abscission, host tissue replacement,
otium. Sclerotia are compact masses of mycelium, often necrosis, and wilt.
spherical or pellet-shaped, that range from 1 mm to 1 cm
in diameter and consist of a central core of hyphae with
Abnormal Growth
lipid and glycogen reserves protected by a thick-walled
rind. Plant pathogens that produce sclerotia or a similar Abnormal growth is caused by hypertrophy (excessive
structure, the microsclerotium, tend to persist many years cell enlargement), hyperplasia (excessive cell division),
in the absence of a suitable host. Sclerotia are common and etiolation (excessive elongation). Tissues affected by
among species in the Ascomycota and Basidiomycota, these processes may be gall- or club-like, misshapen, or
especially among those that infect herbaceous plants as a curled. Symptoms associated with abnormal growth
means of surviving between crops. include the following:
Clubroot – roots are swollen, spindle- or club-shaped.
For example, clubroot of crucifers (caused by
Fungi and the Environment
Plasmodiophora brassicae); the clubs form as cells abnor-
Many factors (moisture, a carbon source and other nutri- mally divide or enlarge.
ents, and the proper environment) are necessary for Etiolation – excessive shoot elongation and chlorosis,
fungal growth. Of these, adequate moisture (free water induced in poor light or by growth hormones. For exam-
as well as high relative humidity) is highly critical. ple, foolish seedling disease of rice (also called bakanae,
Moisture is necessary to prevent desiccation of hyphae, caused by Gibberella fujikuroi). Study of this disease led to
to facilitate nutrient uptake, to facilitate germination and the discovery that plants produce similar compounds
penetration of host tissues during the infection process, called gibberellins.
and for dispersal. Fungi that can adapt to low moisture Gall – many fungi cause galls (enlarged growths,
availability do so by regulating the concentrations of round or spindle-shaped) to form on leaves, stems, roots,
cellular solutes or by producing resting structures with a or flowers. For example, cedar affected by cedar-apple
wall structure that withstands drying. rust (caused by Gymnosporangium juniperi-virginianae); galls
Most plant pathogenic fungi are aerobic, requiring oxy- consisting of both host and fungal tissue develop in stem
gen for respiration (generation of energy). Some fungi such tissues. These galls later serve as a source of inoculum.
Leaf curl – leaves are discolored and curled, often at the Canker – elliptical lesions on branches and stems that
edge of the leaf. For example, species of Prunus affected by destroy vascular tissue; lesions appear cracked, raised,
Taphrina deformans (causal agent of peach leaf curl). sunken, or associated with resin (conifers); affected
Wart – outgrowth on stems and tubers. For example, branches may wilt, die back, and die. For example,
potato wart disease caused by the chytrid Synchytrium Cytospora canker of spruce (Cytospora kunzei, syn.
endobioticum. Leucocytospora kunzei (anamorph), Leucostoma kunzei, syn.
Witches’ broom – profuse branching, resembling a Valsa kunzei (teleomorph)) is one of the most important
spindly broom, is a common symptom associated with diseases of Colorado and Norway spruce in the landscape.
several fungal diseases. For example, witches’ broom of Cankers develop on the lower limbs, killing branches as
cacao (Crinipellis perniciosa). C. perniciosa is a basidiomycete the disease progresses up the tree. As with many canker
found wherever cocoa is grown in the western hemi- diseases, fruiting structures are evident in the killed tis-
sphere. Yields in severely affected plantings may be sue, which is also filled with resin.
reduced by as much as 90%. Cutting rot – cuttings in propagation beds are blackened
from the cut end and up through the stem; ensuing rot
rapidly kills the cuttings. For example, Blackleg of geranium
Abscission
(Pythium spp.). This cutting rot is common in propagation
Abscission includes premature defoliation and fruit drop, beds that are overly moist or poorly drained and contami-
and shot hole (where affected portions of the leaf blade nated with the fungus. Affected plants must be destroyed.
drop out). Many foliar diseases cause premature leaf or Damping-off – seeds and seedlings are killed before
fruit drop. These include apple scab (V. inaequalis), shade (preemergent) or after (postemergent) they break the soil
tree anthracnose (Apiognomonia spp.), and ash rust (Puccinia surface. For example, damping-off of ornamental or vege-
sparganioides). Shot-hole forms on leaves of stone fruit table seedlings caused by species of Fusarium, Pythium, and
when portions of the leaf blade that surround an infection Rhizoctonia. The first indication of damping-off is a bare
site break down, causing the affected portion to shrivel patch (preemergent damping-off), often circular, within a
and drop from the blade. flat of seedlings. As the disease progresses through the flat,
seedlings that have already emerged collapse and die
(postemergent damping-off). The fungi attack these seed-
Host Tissue Replacement
lings at the root tips or at the soil line.
Host tissues, particularly reproductive structures, may be Dieback – twig or branch necrosis that begins at the tip
replaced by fungal hyphae and spores. In the disease ergot and progresses toward the twig base. For example, sudden
of rye and wheat, C. purpurea invades the grain within the oak death and ramorum blight (P. ramorum) is a recently
seed, replacing the embryo with a fungal sclerotium (the identified disease that, in its canker form (sudden oak death),
ergot) that is poisonous to animals and humans. Other has killed over 1 million trees in the Fagaceae (oak family)
examples in this category include the bunts and smuts, in coastal California. The host list for this pathogen includes
characterized as galls or seed heads that are entirely filled many native understory species as well as horticultural
with masses of teliospores. Ears of corn affected by corn crops, such as azalea and rhododendron, where shoot die-
smut (caused by Ustilago maydis) become infected through back is a predominant symptom (ramorum blight).
the silk. The galls that form in place of the kernels Dry rot – a crumbly decay of fleshy plant organs. For
enlarge, eventually filling with hyphae that convert to example, Fusarium dry rot of potato (Fusarium sambucinum)
smutted masses of black teliospores. affects tubers and seed pieces, reducing crop establishment.
Leaf spot or blotch – discrete lesions of dead cells on
leaf tissue between or on leaf veins; appearance varies as
Necrosis
lesion borders and centers differ in color or are target-
The most common reaction to fungal infection is necrosis. like; sometimes accompanied by a yellow halo; fungal
Cells are killed by fungal enzymes, toxins, or host defense fruiting structures are often evident in dead tissue.
responses; affected tissue is brown or blackened, dry or Example of leaf spot is strawberry leaf spot
slimy, sunken, and (in leaves) is often preceded by chloro- (Mycosphaerella fragariae); example of leaf blotch (larger,
sis (yellowing, caused by a breakdown of chlorophyll). more diffuse regions of dead tissue) is horsechestnut leaf
Symptoms associated with necrosis include the following: blotch (Guignardia aesculi).
Anthracnose – sunken or blackened lesions appear on Needle cast – needles of conifers, very often the pre-
leaves (often following leaf veins and/or leaf margins), vious season’s growth, are prematurely cast from the tree.
stems, and fruit. For example, anthracnose (or cane spot) For example, Rhabdocline needle cast (Rhabdocline
of brambles (Elsinoe¨veneta), a serious disease of purple and pseudotsugae) affects only Douglas-fir. In the spring, red-
black raspberries. In severe cases, fungal lesions girdle the brown lesions with fruiting structures form on the pre-
stem to weaken or kill the cane. vious season’s needles. Ascospores are released during
damp weather to infect the new growth. Needles are then fungal genetic sequences or their protein products have
cast from the tree after spore release, resulting in trees become increasingly useful to taxonomists, especially with
that may be unfit for sale. regard to matching an anamorph with its corresponding
Root and crown rot – root necrosis extends from death teleomorph. Methods such as isozyme analysis, restriction
of feeder roots to the entire root system, often extending fragment length polymorphism (RFLP), and random
into the crown to girdle the base of the stem; symptoms amplified polymorphic DNA (RAPD) are used to compare
aboveground include dieback, wilt, and death of the closely related isolates. PCR techniques are also useful for
canopy. For example, Phytophthora root and crown rot detecting pathogens in culture, host plants, or debris.
(also known as Rhododendron wilt) (Phytophthora cinna- Although once classified in the kingdom Planta, the fungi
momi, P. parasitica, and other species) affects the roots of are now placed in three different kingdoms: Protozoa,
rhododendron, azalea, and other ericaceous hosts grown Stramenopila, and Eumycota (also called kingdoms
in soils with excessive moisture. The fungal pathogens Mycota or Fungi). Pathogenic fungi are best identified by
produce zoospores that disperse within the root systems the phylum to which they belong and are placed in these
of susceptible species. The resulting root necrosis causes phyla based on the sexual state of their life cycle. Organisms
aboveground symptoms of wilt (leaves of rhododendron considered true fungi (Eumycota) are classified in the
will roll downward along the midvein), dieback, and die. phyla Chytridiomycota, Zygomycota, Ascomycota, and
Scab – lesions on fruit, leaves, tubers, and other organs Basidiomycota. These organisms are referred to as chytrids,
become crusty, raised, or sunken. For example, apple scab zygomycetes, ascomycetes, and basidiomycetes, respec-
(V. inaequalis) is one of the most serious diseases of apple tively. Fungus-like organisms, which differ in their
and crabapple grown commercially or for landscape use. evolutionary history, are placed in the Protozoa (the
Seriously affected trees defoliate, fruit develop scabbed Plasmodiophoromycota or endoparasitic slime molds) and
lesions and may be unfit for sale, and yield is poor. Stramenopila (the Oomycota, or oomycetes; also referred to
Soft rot – fleshy plant organs (bulbs, corms, fruit, as water mold fungi). The Plasmodiophoromycota also con-
rhizomes, and tubers) are macerated, becoming water tains the slime molds (Myxomycota), which are commonly
soaked and soft; affected tissues lose moisture, becoming seen saprophytes in the landscape but are not plant parasites.
hard or shriveled into mummies. For example, Rhizopus The Chytridiomycota, once considered protozoans, may be
soft rot of papaya (Rhizopus stolonifer) is a common disease regarded as ancestors of the other true fungi. As stated
of papaya that occurs post harvest. Affected papaya fruit previously, the ‘fungi’ as a group fill a similar ecological
in storage rapidly decay, leaving the cuticle intact. The niche, even as their evolutionary backgrounds differ. Basic
fungus emerges from cracks in the cuticle, spreading to differences among these groups are described in Table 1.
other fruit. Fruit are further decayed by other fungi and Below are descriptions of the different phyla within
bacteria, emanating a sour odor. their respective kingdoms and examples of plant pathogens
for each.
Permanent Wilt
Kingdom Protozoa
Permanent wilting occurs when vascular tissues are blocked
or destroyed by fungal growth, toxins, or host defense Plant pathogenic protozoans are classified within the
responses. Tissue connected to affected vessels may wilt Plasmodiophoromycota. These endoparasitic slime
and die, and associated leaves may scorch and prematurely molds are biotrophs, and represent one of several fungi
abscise. A common disease of both herbaceous and woody that produce zoospores, which serve a dispersal (primary
plant species is Verticillium wilt (caused by Verticillium zoospores) as well as a reproductive (secondary zoos-
dahliae). The fungus survives as microsclerotia in the soil, pores) purpose. The thallus of these organisms is a
which germinate in the presence of root exudates. The multinucleate, amoeboid plasmodium that lacks a cell
fungus subsequently invades the xylem, producing conidia wall and is restricted to the host plant cell. Sexual repro-
on verticilliate whorls of sporophores. The ensuing vascular duction in this group includes fusion of zoospores (as
dysfunction is accompanied by symptoms of permanent isogametes) to form a zygote. Species of two genera
wilt, which, in many cases, results in the death of the plant. within this group, Polymyxa and Spongospora, can also vec-
tor plant pathogenic viruses.
Clubroot of crucifers, caused by P. brassicae, is one of
Pathogen Groups the most significant diseases of crucifers (cauliflower and
cabbage) worldwide. This soilborne fungus invades root
Fungal taxonomy has been classically based on the mor- cells, inducing hypertrophy and hyperplasia, to form
phology and genesis of spores, hyphal and colony ‘clubs’ that interfere with normal root function. Affected
characteristics, nutrition, and growth on selective media. plants wilt and stunt. Plasmodiophora survives in soil for
Recent advances in molecular techniques that examine extremely long periods as resting spores (the product of
meiosis); thus, fields with a history of this disease cannot Kingdom Stramenopila
be replanted with susceptible host species.
Organisms in the Stramenopila are considered protists
The disease cycle of clubroot of crucifers begins as
that include the brown algae, diatoms, and water molds.
haploid resting spores in the soil germinate to form pri-
The flagella in this group exhibit hair-like projections, a
mary zoospores. These spores, as agents of dispersal,
characteristic that gives the Stramenopila (Latin: stramen
identify, encyst, and penetrate the root hairs of suscepti-
(flagella) þ pilos (hairs)) its name. Water molds are placed
ble hosts. A primary, multinucleate plasmodium forms as
in the phylum Oomycota.
the nuclei divide through mitosis. The plasmodium even- Plant pathogenic oomycetes are considered ‘water
tually cleaves into multinucleate portions, each molds’ because most species produce motile, biflagellate
developing into a sporangium (zoosporangium) with 4–8 zoospores that require free water for dispersal. The thal-
haploid secondary zoospores. At this stage of the disease, lus of the Oomycota consists of diploid, coenocytic
the impact of the parasite on plant function is slight. hyphae that contain cellulose (a defining characteristic)
Secondary zoospores are discharged to soil through and glucans in the cell wall. The result of asexual repro-
pores dissolved in the root cell wall. These zoospores duction in this group is the zoospore produced in a
may act as isogametes to form diploid cells that penetrate zoosporangium; some species produce additional chlamy-
roots directly or through wounds. The diploid plasmodia dospores, others reproduce asexually by means of
that result from fertilization are much more damaging to nonmotile sporangiospores. Sex in the Oomycota results
the host; the fungus spreads to cells throughout the cortex in a diploid oospore produced when a female gamete
and vascular system, clubs form as a result of hyperplasia (oogonium) is fertilized by a nucleus from the male
and hypertrophy, and the plants wilt and stunt. Meiosis in gamete (antheridium). Since the thallus is diploid, meiosis
plasmodial cells results in the production of resting must occur as the gametes, which are morphologically
spores, which are released into the soil as the roots disin- differentiated, are formed.
tegrate; a group of resting spores is called a sorus. Most plant pathogenic oomycetes are placed in two
Management of clubroot of crucifers includes crop orders (Table 3). The only significant plant parasites
rotation (crucifers no more than once every 3 to 5 years) within the Saprolegniales are species of the genus
and increasing soil pH to 7.2 to inhibit spore germination. Aphanomyces, which causes a root rot of annual plants
Some cultivars are resistant to some races of the pathogen, such as pea and sugar beet. Within the Peronosporales,
but not to all. most pathogenic oomycetes, such as species of Pythium
Table 3 Plant pathogenic oomycetes
Order Characteristics Examples
Saprolegniales Well-developed mycelium, zoosporangium is long and Aphanomyces euteiches (pea)

cylindrical
Peronosporales Well-developed mycelium, zoosporangia oval or
(family): lemon-shaped
Pythiaceae Sporangia borne on hyphae of indeterminate length Phytophthora infestans (late blight of potato and
tomato)
Phytophthora palmivora (black pod of cacao)
Phytophthora nicotianae (black shank of tobacco)
Phytophthora ramorum (sudden oak death,
ramorum blight)
Pythium aphanidermatum (damping-off and root rot
of many hosts; root rot of cucumber)
Pythium debaryanum (damping-off)
Pythium ultimum (damping-off; black leg of
geranium)
Peronosporaceae Sporangia borne on hyphae of determinate length Bremia lactucae (lettuce)
(downy mildew) Peronospora destructor (onion)
Peronospora lamii (coleus)
Peronospora tabacina (blue mold of tobacco)
Plasmopara viticola (grape)
Pseudoperonospora cubensis (cucurbits)
Albuginaceae (white Sporangia formed in chains on club-shaped Albugo candida (crucifers)

rust) sporangiophores of indeterminate length Albugo ipomoeae-panduranae (sweet potato)
and many species of Phytophthora, are soilborne and attack growing areas to remove any traces of contaminated soil.
roots. Others, such as the downy mildews, the white rusts, Pasteurized growing medium (often with inhibitory
and other species of Phytophthora, are associated with amendments such as composted tree bark) is used, and
aerial plant parts. overwatering is avoided. Chemical seed treatments and
Pythium diseases are common and can be very dama- preventive fungicides are available for use, and resistant
ging to crops during production, transit, post harvest, and cultivars of some commercially grown plants may be
at market. Pythium species are ubiquitous, causing a damp- selected by growers in lieu of more susceptible varieties.
ing-off, seed and root rot, or cutting rot of all plant types. Phytophthora species cause diseases on a variety of hosts,
A soft rot can also occur when fleshy organs (such as fruit including seedlings, annual plants, herbaceous perennials,
and vegetables) come in contact with the soil. These shrubs, and fruit and forest trees. These diseases are
oomycetes are especially troublesome in greenhouse characterized as damping-off, root rot, rot of the crown,
plants and in turfgrasses, and in general, younger plants stem, tuber, corm, bud, or fruit, foliar blight, stem canker,
are more susceptible than older plants. and dieback. Species of Phytophthora may be host specific
The Pythium infection process begins when germ tubes or may have a broad host range. For example, a newly
from encysting zoospores or from germinating sporangia described disease caused by P. ramorum (sudden oak death
directly penetrate susceptible plant tissues. The fungus or ramorum blight) has, to date, a broad host range of
grows between and within host cells, producing pectinase more than 100 species. Symptoms include bole cankers on
to dissolve pectin in the middle lamella and cellulase to certain oak species as well as a variety of leaf spots, leaf
break down cell walls. The result is a rotted mass of blights, and shoot blights on many ornamental hosts, some
macerated tissue. As colonization progresses, sporangia of the natural understory plants associated with suscep-
produce a balloon-like vesicle from which zoospores are tible forest trees.
released. Chlamydospores, as well as oogonia, also form. Late blight of potato, a disease caused by Phytophthora
At the appropriate time, oospores, which serve a survival infestans (meaning ‘infectious plant destroyer’), has special
purpose for the fungus, may germinate by producing a historical significance. Controversy over the etiology of
germ tube or by releasing zoospores from a vesicle late blight (which caused the Irish potato famine in the
(Figure 1). mid-1840s) eventually led to the first accepted experi-
Management of diseases caused by Pythium species is mental proof that microorganisms, in this case fungi, are
very important in propagation operations where highly the cause of disease and not, as surmised at the time, the
vulnerable plants may be introduced to fungal propagules. result of wet weather or the wrath of God. The story
In these cases, care is taken to clean propagation and begins in the late 1500s when the potato was brought
(f)
(c)
(d)
(e)
(b)
(a)
Figure 1 Reproduction in Pythium aphanadermatum (Oomycota), a root, stem, seed, and fruit rot pathogen with a very wide host
range. (a) and (b) Sexual stage. Oogonium (female gamete) with an (a) intercalary or (b) a terminal antheridium (male gamete).
(c–f) Asexual stage. (c) Inflated, lobate sporangium. (d) Sporangium with vesicle. (e) Sporangium with vesicle containing zoospores.
(f) Biflagellate zoospores. Reproduced from Shishkoff N, with permission from CRC Press.
from its center of origin in Central America to Europe by infected, resulting in up to 90% crop loss in severe cases.
Spanish conquistadors. By the 1800s, potatoes were The most significant downy mildews are those that infect
widely planted throughout Europe and North America, cucurbits (Pseudoperonospora cubensis), grape (Plasmopara
although genetic variation within the genus was limited. viticola), onion (Peronospora destructor), and tobacco (blue
Eventually, a pathogen of potato was introduced with mold, Peronospora tabacina).
potatoes from South America to potato-growing regions The ‘downy’ growth associated with infected tissues,
elsewhere. The ensuing disease in the 1840s, the result of from which the disease gets its name, consists of charac-
a combination of susceptible potatoes grown in monocul- teristic, dichotomously branched stalks (sporangiophores)
ture, weather favorable for disease development, and a that emerge through the stomates on the lower leaf sur-
virulent pathogen, proved catastrophic to cultures, and face. Sporangia form at the tips of the sporangiophores
especially Irish peasants, who depended heavily on the and are dispersed by wind or water to new hosts.
potato for sustenance. Sporangia may germinate directly to infect new plants
P. infestans affects aerial plant parts (leaves and stems) or, in cooler weather, produce zoospores, which rapidly
as well as tubers. This oomycete overwinters within facilitates disease spread. The pathogen overwinters as
infected tubers. When tubers are used as seed pieces in oospores in infected plant material or in soil, depending
the spring, the fungus infects the developing seedling, on species. Free moisture, high relative humidity, and
emerging from infected tissues as a white mildew of cool or warm, but not hot, weather enhance disease
hyphae. Sporangia produced on hyphal tips are dispersed development.
by air currents to aboveground plant parts or may be Downy mildew of grape was introduced from the
washed into the soil. These propagules form a germ United States to European vineyards of Vitis vinifera in
tube that penetrates host tissue directly or through sto- about 1875. Although the fungus does little damage to the
mates. In slightly cooler weather, each sporangium grapes native to North America, V. vinifera is highly
instead releases eight biflagellate zoospores that serve as susceptible, and the grape and wine industry throughout
propagules. Within a few days, necrosis occurs on the France and much of Europe was nearly destroyed. Studies
affected plant part, shortly accompanied by sporangial of this disease in France led to the discovery of one of the
development. In this manner, many generations of asexual first fungicides, bordeaux mixture (copper sulfate and
spores are produced within a single growing season. hydrated lime). Most varieties of V. vinifera are still highly
Foliage affected by late blight becomes brown, water susceptible to the disease, and fungicides remain an
soaked, and covered with white fungal mycelium. Plants important tool for management of this disease.
may die in the field, severely reducing yield. Tubers
infected with the pathogen develop brown and purple
Kingdom Eumycota (Fungi)
splotches that appear water soaked, later drying to
become shrunken and firm. Affected tubers may rot in Chytridiomycota
storage and cannot be used as seed pieces the following Fungi within the Chytridiomycota, called chytrids, inha-
season. bit water or soil and are the oldest known true fungi.
P. infestans is heterothallic; thus, sexual reproduction Chytrids lack a true mycelium. The thallus is irregularly
requires two mating types (A1 and A2) for production of shaped and the cell wall, as in other true fungi, contains
oospores. Until the 1980s, sexual reproduction was unre- chitin and glucans. Unlike other true fungi, however,
ported outside of Mexico. Since then, both the mating chytrids produce motile zoospores that possess a single,
types have been identified from some potato-growing posterior, whiplash flagellum.
regions, including the United States, and several more Members of the Chytridiomycota are mostly sapro-
virulent strains of the fungus have emerged. phytes; the few known pathogens of vascular plants in this
Management of the disease requires the use of seed pieces group include Olpidium brassicae (a root pathogen of cab-
that are free of the disease as well as preventive fungi- bage and other hosts), Physoderma alfalfa syn. Urophlyctis
cides. Field resistance to P. infestans among commercial alfalfae (crown wart of alfalfa), Physoderma maydis (brown
potato cultivars varies; many of the most popular cultivars spot of corn), and S. endobioticum (black wart of potato).
are susceptible. Environmental conditions have a great The wart-like (gall) symptoms induced by Physoderma and
impact on disease development; in cool, wet weather, Synchytrium occur as cells in affected tissues are stimulated
the disease persists, killing plants in a matter of days. In to divide repeatedly. The brown of corn pathogen affects
drier conditions, the progress of the disease is diminished. aerial plant parts; severe infection results in stalk rot and
Downy mildews are caused by a handful of obligate lodging in the field.
parasites that occur on many cultivated crops. These O. brassicae is a symptomless parasite and is probably
oomycetes are aerial pathogens that cause necrosis on most significant because it vectors plant viruses such as
foliage, stems, and fruit. Infected plants may stunt, defoli- tobacco necrosis virus and lettuce big vein virus. The
ate, decline, and die. Younger plants may be systemically pathogen is endobiotic: the entire thallus resides within
a single host cell. The disease cycle begins when the nutrients released are used by the fungus to produce fluffy
thick-walled resting spores germinate to form zoospores, tufts of a gray/brown aerial mycelium consisting of fungal
which during the penetration process dissolve a small hyphae and fruiting structures. Eventually, moisture is
pore in the cell wall of root epidermal cells or root hairs. lost from the degraded tissue, which becomes firm and
The zoospore protoplast enters the cell and grows to form mummy-like.
a zoosporangium. As the zoosporangium forms, nuclei
from repeated mitotic divisions are packaged into zoos-
pores; as such, the entire thallus is converted into an Ascomycota
asexual reproductive structure (i.e., holocarpic). When The Ascomycota are some of the best-known true fungi,
the roots are wet, zoospores escape through an exit pore and at least 30 000 different species of ascomycetes are
to infect nearby cells. The thallus may also convert into a described. The group is very diverse and occupies a
thick-walled resting structure that can remain viable in variety of niches. A minority of these fungi form partner-
the soil for many years. The presence of a sexual cycle in ships with algae to form lichens; the remainder are
this organism has not been confirmed. saprophytes or symbionts. Parasitic ascomycetes may
derive nutrition as biotrophs, necrotrophs, or hemibio-
Zygomycota trophs. Although some ascomycetes, such as yeasts, have a
The Zygomycota are terrestrial fungi with a well- single-celled thallus, the thallus of most of these terres-
developed, coenocytic, haploid mycelium. The thallus is trial fungi consists of a well-developed, septate, haploid
haploid, and chitin and chitosan are significant constitu- mycelium that contains chitin in the cell wall. The asco-
ents of the hyphal cell wall. Asexual reproduction in the mycete thallus grows under the substratum surface; only
zygomycetes results in nonmotile spores called sporan- reproductive structures are exposed to the air.
giospores. Sexual spores, or zygospores, are produced Ascomycetes are named after the ascus, a sac-shaped
when two morphologically similar gametangia of opposite structure that contains ascospores, the products of meiosis
mating types fuse. These fungi are saprophytes or weak during the sexual reproductive process. Asci are formed
pathogens, causing postharvest molds and soft rots. For when the female sex cell (ascogonium) is fertilized by the
example, some species of Mucor are soil inhabitants that male gamete (antheridium). The diploid zygote nucleus
penetrate fruit (through wounds or at the calyx) that have undergoes meiosis followed by one mitotic division to
fallen to the orchard floor. Within two months of cold form eight ascospores, which remain in the sac until
storage, the fruit are completely decayed and fungal they are discharged and disseminated.
mycelium emerges in tufts through the cuticle. Asci are unitunicate (with a single wall) or bitunicate
Although R. stolonifer is best recognized as the common (with a double wall). For the majority of ascomycetes, asci
bread mold, under the right circumstances, this ubiqui- are produced in fruiting structures called ascomata (or
tous saprophyte also causes a soft rot of fleshy fruit and ascocarps). The different types of ascomata are the
vegetables, bulbs, corms, flowers, and seeds. The thallus apothecium (open, cup-shaped with exposed unitunicate
consists of hyphal structures known as rhizoids (short asci); cleistothecium1 (completely closed, lined with one
branches of hyphae that resemble roots), which penetrate or more unitunicate ascus); perithecium (flask-shaped
the food substrate, and stolons (longer branches of with an opening, or ostiole, at the tip, lined with unituni-
hyphae) that skip over the substratum surface. The fungus cate asci); and pseudothecium (or ascostroma; bitunicate
produces asexual sporangiospores (also called mitospores) asci are produced in a cavity or locule buried within a
that form on the swollen tip (columella) of a long aerial stroma of fungal mycelium). (Historically, the ascoma
sporangiophore. The fungus is heterothallic; as its food associated with powdery mildews has been called a cleis-
supply is depleted, isogametes of opposite mating type tothecium because, at least initially, the sporocarp is fully
fuse to form a zygosporangium containing a single het- closed. Later, however, these structures develop a line of
erokaryotic zygospore. After a 1–3 month period of weakness to break open during ascospore release. In other
dormancy, the zygospore undergoes meiosis to form texts, these structures are defined as chasmothecia.
four haploid nuclei (two of each mating type) and germi- Although, developmentally, ascoma produced by pow-
nates to form a sporangium. Sporangiospores are formed dery mildews may be more similar to perithecia, the
by mitosis; these spores are homokaryotic; one-half of the previously accepted term ‘‘cleistothecium’’ is used here.)
spores are of one mating type, and the other half are of the Those asci that are formed freely without a supporting
opposite mating type. fruiting structure (e.g., for leaf curl fungi and yeasts) are
The soft rot disease process begins as sporangiospores, called naked asci.
ubiquitous in the air, penetrate through wounds in various Asexual reproduction in ascomycetes is most common
plant parts. Cellulase and pectinase enzymes, produced as conidia produced on conidiophores; other forms
by the fungus, degrade the middle lamella and cell walls include chlamydospores and reproduction by budding
of plant tissues, causing a soft, water-soaked rot. The or fission (yeasts).
One of the most notable diseases caused by an asco- disease develops, ergots form in place of kernels. The
mycete is ergot of rye and wheat, caused by the ergots mature at the same time as the grain, and are
perithecial fungus C. purpurea (Table 4). Grain in the harvested or fall to the ground. Since these sclerotia lose
seed head is replaced by a survival structure of the fungus, viability after a year, management of this disease includes
called an ergot (sclerotium). Ergots are poisonous to deep plowing and crop rotation, as well as use of patho-
humans and animals and, when ingested with flour made gen-free seeds. It is not permissible to mill flour that
from contaminated rye or wheat, affect the nervous sys- contains more than 0.3% by weight of sclerotia.
tem and restrict blood vessels. Side effects of the disorder Apple scab, one of the most important diseases of
in humans, known as ergotism, holy fire, or St. Anthony’s apple, crabapple, and other rosaceous species worldwide,
fire, include convulsions, gangrene, hallucinations, and is caused by V. inaequalis. Symptoms of this disease
miscarriage. Convulsive hallucinations caused by ergot include lesions on the foliage (olive-green spots with
were implicated in the Salem witch trials, but the rela- feathered margins), premature leaf and fruit drop, scab-
tionship remains unproven. Interestingly, LSD (lysergic bing and cracking of fruit, poor bud set, and reduced yield.
acid diethylamide) was first isolated from the sclerotia of The disease was likely introduced to North America in
this fungus. the 1600s as European colonies planted infected apple
C. purpurea (ana. S. segetum) occurs worldwide and is a trees and scions. The first botanical description of symp-
more serious pathogen of rye than of wheat or other toms was made by Elias Fries in Sweden in 1819, and the
cereals. In the spring, sclerotia associated with fallen pathogen itself was described by Cooke in 1866. Once
seed heads germinate to form many perithecia on stalks considered an accepted fact of life, apple scab can be
along the sclerotium periphery. Each perithecium con- debilitating to susceptible commercial apple and orna-
tains many asci, which in turn contain eight multicellular mental hosts.
ascospores. The ascospores are disseminated by wind to Apple scab first develops soon after a budbreak as ascos-
infect the ovaries of developing flowers. Within a week, pores are forcibly ejected from pseudothecia embedded in
droplets of conidia exude from the infected florets in a infected plant material (leaf litter) on the ground. Each
matrix of sticky liquid called honeydew. Insects, attracted pseudothecium contains up to 100 asci. The ascospores
to the honeydew, carry conidia from flower to flower. are carried by air to the developing leaves and fruit on the
Conidia are also dispersed by splashing rain. As the tree. Ascospore release (as primary inoculum) continues for
Table 4 Plant pathogenic ascomycetes
Class Ascoma Examples
Archiascomycetes Naked Taphrina caerulescens (oak leaf blister)

Taphrina deformans (peach leaf curl)
Leotiomycetes Cleistothecium Tribes (genus):
(Erysiphales) Erysiphae: Erysiphe (herbacious plants), Microsphaera (lilac), Uncinula (grape)
Phyllactinieae: Phyllactinia (shade trees), Leveillula (tomato)
Golovinomyceteae: Arthrocladiella (boxthorn), Golovinomyces (Asteraceae,
coreopsis)
Cystotheceae: Podosphaera (apple, cucurbits, rose)
Blumerieae: Blumeria (cereals)
Pyrenomycetes Perithecium Ceratocystis fagacearum (oak wilt)
Claviceps purpurea (ergot of rye and wheat)
Gaeumannomyces graminis (take-all of wheat)
Gibberella zeae ana. Fusarium graminearum (head blight of wheat)
Glomerella cingulata (bitter rot of apple and pear)
Monosporascus cannonballus (root rot and vine decline of melon)
Nectria spp. (canker of hardwoods)
Ophiostoma ulmi; Ophistoma novo-ulmi (Dutch elm disease)
Loculoascomycetes Pseudothecium Apiosporina morbosa (black knot of plum)
Elsinoë ampelina (grape anthracnose)
Guignardia bidwellii (black rot of grape)
Mycosphaerella musicola; M. fujiensis (Sigatoka disease of banana)
Venturia inaequalis (apple scab)
Discomycetes Apothecium Diplocarpon rosae (black spot of rose)
Lophodermium spp. (needle cast of pine)
Monilinia fructicola (brown rot of stone fruit)
Rhabdocline pseudotsugae (Rhabdocline needlecast of Douglas-fir)
Rhytisma acerinum; R. punctatum (tar spot of maple)
a period of up to 9 weeks, through the period the host is Sphaerotheca) pannosa f. sp. rosae, also overwinter in buds.
most vulnerable to infection. Penetration of new leaves In this species, primary inoculum is produced in spring,
requires a period of leaf wetness that varies from 9 to 28 h, when mycelium in buds infects developing tissue or when
depending on temperature. A fungal germ tube from the ascospores developed within cleistothecia are released.
germinating ascospore penetrates the cuticle, and a myce- Powdery mildews are classified in one of the five tribes
lium develops between the epidermis and the cuticle. (Table 4). Characteristics of the anamorph (conidial
Conidia are produced on sporophores that push through chains and surface ornamentation) as well as cleistothecial
the cuticle in mats. These asexual spores (as secondary morphology (number of asci per ascocarp, morphology of
inoculum) are released through the remainder of the grow- hyphal appendages on the cleistothecium) are useful
ing season. At leaf fall, the fungus grows deeper into the taxonomic criteria.
mesophyll of infected leaves, forming pseudothecia. The Management of powdery mildews includes the use of
fungus overwinters in this condition until the following resistant cultivars when available. Unfortunately for rose
spring, when the disease cycle starts anew. growers, most popular cultivars are susceptible to this
Management of apple scab is extremely important for disease. The powdery mildew of wheat pathogen is easily
commercial growers, and usually requires a multifaceted adaptable and has many races, so breeders strive to incor-
approach. Cultural management techniques include porate as many genes for resistance as possible into wheat
proper orchard placement and tree orientation, manage- cultivars to increase their durability. Best practices for this
ment of humidity and leaf wetness, the use of resistant disease include rotating crops, removing volunteer plants,
cultivars where available, removal of fallen leaves if prac- and mixing wheat cultivars with different genes for resis-
tical, and shredding leaf litter with a mower, adding urea tance within a single planting. For most powdery
to hasten decomposition. Growers usually rely, however, mildews, humidity control through proper spacing and
on fungicide sprays for effective disease control. The weed control is helpful, as is the use of contact or systemic
target of chemical control is to effectively control the fungicides. Unlike many other diseases, nontraditional
release of primary inoculum (ascospores) at the beginning control products (such as oils, potassium bicarbonate,
of the season. Calendar sprays, used in the past, are based dilute hydrogen peroxide, and biological controls) are
on the phrenology of the host tree. Currently, more useful for powdery mildew management, presumably
sophisticated forecasting systems, based on temperature because the thallus of the fungus is on the plant surface
and rainfall, are in use. As a result, fewer better-timed and is easy to eradicate. These products offer attractive
sprays are made. alternatives to fungicides for the landscape or residential
Powdery mildews are biotrophs, named after the clientele.
mats of mycelium and spores evident on the plant Brown rot of stone fruit (e.g., peaches, cherries, plums,
surface. They are the most common, and probably the and almond) is caused by several different species of
best recognized, of all plant diseases. Although all kinds the discomycete Monilinia (M. fructicola, M. laxa, and
of plants are affected by powdery mildew, the greatest M. fructigena). Losses from this disease occur both in the
economic impact probably occurs on cucurbits and field and post harvest. Brown rot manifests as a blossom
cereals. Powdery mildew fungi seldom kill their hosts, and fruit blight. Cankers may also develop on twigs and
but diminished photosynthetic capacity resulting in branches. Fruit affected by this disease develop brown
impaired growth and reduced yield may occur in spots that quickly consume the fruit. Tufts of gray/brown
some susceptible species. For example, severe disease mycelium break through the cuticle, and the fruit even-
caused by powdery mildew of wheat (caused by tually loses moisture and mummifies. The sexual stage
Blumeria graminis f. sp. tritici) results in significant develops on mummified fruit that drop to the orchard
yield loss and lodging in the field. floor and become partially buried. As many as 20 apothe-
As biotrophs, powdery mildew fungi draw nutrients cia, each lined with thousands of asci, develop per fruit.
from epidermal cells via haustoria that penetrate through Management is best achieved by controlling the blossom
the cell wall. All mycelial and spore development occurs blight phase of the disease.
on the plant surface. Throughout the growing season,
powdery mildew fungi produce asexual spores (conidia, Basidiomycota
some egg-shaped) in chains on short conidiophores on the The Basidiomycota are an interesting, diverse group of
plant surface. Conidia are disseminated by air currents to terrestrial fungi that include decay organisms such as
new hosts (secondary cycle). When humidity is high, white and brown (wood) rot fungi, and symbionts such
conidia germinate to initiate new infections. At the end as parasites and mycorrhizal fungi. Many basidiomycetes
of the growing season, host and environmental factors often produce large, spectacular fruiting structures; others
trigger the sexual cycle. Cleistothecia serve as survival are known for their hallucinogenic properties. Like asco-
structures through the winter; some fungi, such as the mycetes, basidiomycetes occupy a variety of niches.
powdery mildew of rose fungus, Podosphaera (sect. Mushrooms are classified in this phylum, as are highly
Table 5 Plant pathogenic basidiomycetes
Order Characteristics Examples
Holobasidiomycetes Basidia club-shaped and single-celled, arranged on an Armillaria mellea (shoestring root rot)
exposed hymenium; includes the gilled mushrooms, boletes, Crinipellis perniciosa (witches’ broom of
polypores, puff-balls, bird’s nest fungi cocoa)
Heterobasidion annosum (heart and butt
rot of conifers)
Heterobasidiomycetes Germinating basidiospores can form secondary spores or Rhizoctonia solani, tel.
yeast-like cells Thanatephorus cucumeris (damping-off,
root disease; aerial blight)
Rhizoctonia cerealis, tel. Ceratobasidium
cornigerum (sharp eyespot of cereals)
Urediniomycetes Rust fungi; complex life cycle consisting of up to five different Cronartium ribicola (white pine blister rust)
spore stages on two unrelated plant hosts Gymnosporangium spp. (cedar-apple
rust, quince rust)
Hemileia vastatrix (coffee leaf rust)
Puccinia graminis (stem rust of wheat)
Ustilaginomycetes Smut fungi; basidia produce numerous basidiospores in Exobasidium vaccinii (azalea leaf and
smutted groups flower gall)
Tilletia caries (common bunt)
Tilletia indica (Karnal bunt)
Ustilago avenae (loose smut of cereals)
Ustilago maydis (corn smut)
important plant pathogens, the rust and smut fungi presence of clamp connections, a bridge that forms during
(Table 5). hyphal development that serves to maintain the dikaryo-
Basidiomycetes have a well-developed septate myce- tic condition of each hyphal cell. In addition, the septa of
lium that has chitin in the cell wall. These organisms may both primary and secondary mycelia contain a special
spend a majority of their life cycle as dikaryotes, where pore (dolipore) so that cytoplasmic continuity is main-
each cell contains two different haploid nuclei. tained between cells. The dolipore septum is a
Basidiomycetes are named after the basidium, a club- characteristic unique to basidiomycetes.
shaped structure upon which (usually) four haploid basi- The holobasidiomycetes are probably best represented
diospores (the result of karyogamy and meiosis in the by the classic mushroom. The mushroom basidiocarp
sexual reproductive process) are perched. Basidia are consists of a cap (pileus) and a stalk (stipe). The pileus is
often arranged in a single layer, or hymenium. The basi- lined with gills, which in turn are lined with basidia. The
diospores are supported by short pointed stalks called stipe of some basidiocarps has a volva (a remnant of the
sterigmata. In most taxa, basidiospores are forcibly ejected universal veil that once enveloped the entire developing
from the basidium. The form of the basidium varies by mushroom) at the base. An annulus (a ringed remnant of
taxonomic group; some basidia are produced on fruiting the partial or inner veil that once enveloped the gills on
structures called basidiocarps. Asexual reproduction in the underside of the basidiocarp) may also be present on
basidiomycetes occurs as conidia (monokaryotic or dikar- the stipe, just under the cap.
yotic) produced on conidiophores, chlamydospores, and A. mellea is both an effective decay organism in the
oidia (monokaryotic thin-walled spores formed by hyphal forest ecosystem and a pathogen, causing shoestring root
fragmentation). Some fungi in the Basidiomycota produce rot or honey mushroom root rot. The fungus, which is
rhizomorphs as structures for survival and dispersal. actually a collection of several species, affects hundreds of
The life cycle of basidiomycetes begins as haploid fruit trees, shrubs, shade trees, and vegetables worldwide.
basidiospores develop into a mycelium with one haploid Armillaria forms characteristic rhizomorphs and sheets of
nucleus per cell. These monokaryotic hyphae are the bioluminescent mycelium (mycelial fans) under the bark
primary mycelium. The dikaryotic hyphal condition is near the crown of infected trees. This white mycelium
established in basidiomycetes when two primary mycelia destroys the phloem and cambium of the tree. The disease
fuse in the first step of fertilization (plasmogamy) (kar- primarily spreads as rhizomorphs extend to adjacent
yogamy is delayed until the basidium is formed). In most hosts. Symptoms exhibited by plants with this disease
basidiomycetes, primary mycelia must be of opposite are similar to those caused by other root diseases, includ-
mating type (heterothallic). It is the secondary mycelium ing reduced growth, small yellow leaves, dieback, and
that supports the development of the basidium. A fre- gradual decline and death. Clumps of basidiocarps
quent characteristic of the secondary mycelium is the (honey-colored mushrooms) are produced at the base of
dead or dying trees in early fall. The fungus continues to growth) and thatch, increasing drainage, and applying
grow as a saprophyte after the plant is dead. fungicides.
From a pathogenic standpoint, the heterobasidiomy- Plant pathogenic rust fungi are Urediniomycetes,
cetes are important because an anamorph of several placed within the order Uredinales. This order contains
teleomorphs in this group, Rhizoctonia sp., is such a sig- some of the most destructive pathogens of vascular plants.
nificant pathogen. Species in this genus (as well as in the Rust diseases have caused famines and ruined the econo-
genus Sclerotium) were classified for many years in an mies of entire civilizations. Crops particularly affected by
artificial group called ‘mycelia sterilia’ because sexual or rust diseases include bean and soybean, grains (barley, oat,
asexual spores were lacking. They are now known to be wheat), asparagus, cotton, pine, apple, coffee, and a vari-
the anamorphs of several basidiomycetes, as well as a few ety of ornamental plants. Rusts are biotrophs, and each
ascomycetes, teleomorphs. species has a relatively narrow host range.
Species of Rhizoctonia occur worldwide. These patho- The generic term ‘rust’ pertains to the rusty spores
gens affect most crops, causing damping-off, root and produced in pustules by some of these fungal species.
stem rot, stem canker, storage rot, and an aerial (foliage) Rusts most often appear on leaves and stems as spots or
blight that occurs when foliage touches the soil surface. lesions that rupture the epidermis, producing rust-
The hyphae of Rhizoctonia branch at right angles, a septum colored, orange, yellow, or white spores. Other rusts
forming just beyond the branch. This characteristic facil- cause galls or swellings to form. The rust fungi are fabu-
itates their identification in culture. Some of these fungi lously complex; their life cycle can consist of up to five
are multinucleate (teleomorph Thanatephorus) and some different spores (pycniospores, aeciospores, uredinios-
are binucleate (teleomorph Ceratobasidium). Rhizoctonia pores, teliospores, and basidiospores) that develop on
solani (multinucleate) is actually a collective species con- one of two unrelated plant hosts (alternate hosts). All
sisting of several unrelated strains. rust fungi produce at least two spore types: teliospores
Strains of Rhizoctonia are distinguished by anastomosis and basidiospores. Disease cycles that contain all five
groupings. Anastomosis occurs when the hyphae of two spore stages are macrocyclic (long cycle); those that con-
isolates fuse and undergo plasmogamy. This process per- tain less than five (usually the urediniospore stage is
mits the development of hyphal networks. Anastomosis is lacking) are demicyclic; those that produce only telios-
successful only between hyphae of the same anastomosis pores and basidiospores are microcyclic. A summary of
group. A killing reaction occurs when these strains belong these spore stages, as well as the Roman numerals
to different anastomosis groups. The anastomosis groups assigned to each stage (a convention used to describe
are not entirely host specific, but some tendencies do each stage in the rust life cycle), is presented in Table 6.
exist. For example, strains of AG1 cause seed rot and Spore stages are produced in sequence. For those fungi
web blight; of AG2 canker of root crops and with two hosts (heteroecious), the principal host is that
brown patch of turfgrass; of AG3 potato diseases; and of which supports the development of the dikaryotic (sec-
AG4 hypocotyl rots on angiosperms. Many species of ondary) mycelium (also called the telial host). Spores
Rhizoctonia produce sclerotia, which remain in soil for up produced by the secondary mycelium are urediniospores
to 3 years. This feature makes control of these organisms and teliospores. The monokaryotic (or primary) myce-
in the field very difficult. lium develops on the alternate host; spores produced on
Brown patch is a common disease of turfgrass caused this host (called the aecial host) are pycniospores and
by R. solani. The fungus survives as sclerotia in plant aeciospores. Diseases that lack an alternate host are auto-
debris, which germinate and grow saprophytically until ecious. Significant diseases caused by heteroecious
hyphae can penetrate a suitable host. Circular lesions macrocyclic rusts include stem rust of wheat (P. graminis),
appear on infected leaves, and purplish patches of dis- ash rust (P. sparganioides), and white pine blister rust
eased plants develop in the turfgrass stand. Management (Cronartium ribicola). Heteroecious, demicyclic rusts
of brown patch includes avoiding excessive nitrogen fer- include those caused by the genus Gymnosporangium,
tilizer (to reduce development of susceptible, succulent such as cedar-apple and quince rusts. Coffee rust
Table 6 Spore stages associated with rust fungi
Spore stage Fruiting structure Spore name Ploidy Parent mycelium Host (if heteroecious)
0 Pycnium (spermagonium) Pycniospore (n) Primary Alternate, or aecial

I Aecium Aeciospore (n þ n) Primary Alternate, or aecial
II Uredinium Urediniospore (n þ n) Secondary Primary, or telial
III Telium Teliospore (n þ n): karyogamy, (2n) Secondary Primary, or telial
IV Basidium Basidiospore (n)
(Hemileia vastatrix) is an autoecious, demicyclic rust, and alternate (or perhaps, less economically useful) host. For
pine-pine gall rust (Endocronartium harknessii) is an auto- most rust diseases, this strategy is not useful since spores
ecious, microcyclic rust. can travel many miles on air currents. In response to a
Stem rust of wheat is a major disease of wheat and wheat rust epidemic in 1916, however, an eradication
barley, causing losses of $5 billion per year worldwide. program was enacted in the United States in 1918 to
The fungus overwinters as diploid teliospores in telia (III) remove barberry plants from wheat-growing regions. By
on infected wheat debris. In spring, teliospores form fila- 1941, 295 million barberry plants were destroyed. The
mentous basidia (IV). Meiosis occurs, and four haploid program should have worked, but later it was found that
basidiospores (produced on short sterigmata) are released the fungus overwinters as urediniospores in the southern
into the air. These spores are dispersed several hundred United States and Mexico. These spores blow up into the
meters by air currents and are deposited on young bar- wheat-growing regions of the United States yearly in
berry leaves (alternate, or aecial host), where they what is known as the Puccinia pathway. Today, barberry
germinate, penetrate the epidermis, and grow intercellu- species planted are resistant to the disease.
larly. Within 3–4 days, flask-shaped pycnia (0) develop, The order Ustilaginales within the Ustilaginomycetes
the tips of which rupture the leaf surface. Haploid- includes the smut and bunt fungi, which are very similar
receptive hyphae (these can be considered female gametes) to rusts in that teliospores and basidiospores are pro-
extend through the opening of the pycnium, accompanied duced, but are dissimilar in that basidia produce
by pycniospores (male gametes), which are extruded numerous basidiospores in smutted groups, which are
through the opening in honeydew. Insects, always attracted often distasteful to look at and smell. There are about
to honeydew, visit the pycnia, become smeared with 1200 species of smuts and bunts distributed worldwide.
pycniospores, and carry them to the receptive hyphae of These destructive pathogens, second only to rusts, can
other pycnia of opposite mating type (heterothallic). severely stunt hosts, reducing yield. Most smuts attack the
Pycniospores are also spread by rainwater or dew. The ovaries of grains and replace the kernel with fungal myce-
receptive hyphae of the opposite mating type are fertilized lium and spores. Others attack leaves, stem, or floral parts.
(plasmogamy), resulting in a dikaryotic mycelium, where The Ustilaginales are biotrophs.
each nucleus is of the opposite mating type. Smut and bunt fungi overwinter as teliospores on
Elsewhere on the leaf during the fertilization process, contaminated seed, in plant debris, or as mycelium within
aecium primordia (aecial mother cells, also haploid) form infected kernels or plants. These fungi are microcyclic,
near the lower leaf surface. Should fertilization occur, the and basidiospores are the only infective propagule. Smuts
aecium primordia are converted to the dikaryotic state by are monocyclic, producing one generation of inoculum
a process called dikarytization, which forms the next per year.
fruiting structure, the aecium (I). Aecia protrude substan- Corn smut, caused by Ustilago zeae, occurs wherever
tially from the lower leaf surface and release dikaryotic corn is grown. Galls form on aboveground plant parts,
aeciospores in chains, which are blown by the wind in late including ears, tassels, stalks, and leaves. Losses of suscep-
spring to nearby wheat plants. Aeciospores penetrate the tible variety in the field can approach 100%. The fungus
stomates of wheat stems, leaves, or leaf sheaths. The overwinters as teliospores in crop debris and in soil where
mycelium grows intercellularly and forms a mat just it remains viable for several years. In spring and summer,
below the epidermis. Dikaryotic urediniospores (repeat- karyogamy occurs and diploid teliospores germinate to
ing spores) push through the epidermis on short produce haploid basidiospores, which are carried by air
sporophores to form uredial pustules or uredinia (II). currents or splashed to young, developing tissue of corn
These spores are blown by air currents up to several plants. Basidiospores germinate and penetrate epidermal
hundred kilometers from the point of origin to reinfect cells directly, forming a fine primary mycelium. Primary
wheat through stomates in the presence of a film of water mycelia of opposite mating type (heterothallic) must fuse
or at a high relative humidity. New urediniospores are to form a dikaryotic secondary mycelium before parasit-
produced every 8–10 days in a secondary cycle. At plant ism can continue. The mycelium grows into surrounding
maturity, uredinia convert into telia (III) and produce plant tissues, stimulating nearby cells to enlarge and
dikaryotic teliospores instead of urediniospores. In addi- divide into galls. Just before sporulation, the galls are
tion, new telia may develop from recent urediniospore invaded by the secondary mycelium, which converts
infections. Karyogamy occurs in the teliospores, which completely into teliospores. Galls turn black and rupture,
then overwinter in the diploid state. exuding a smutted mass of teliospores.
Management of stem rust includes the use of resistant Management of corn smut often includes removal of
and earlier maturing varieties. Although fungicides such infected ears before teliospores are released. Crop rota-
as sterol or demethylation inhibitors can be effective, they tion, deep plowing of stubble in the fall, and use of
are often cost prohibitive to apply and are not used. resistant varieties is recommended. Although there are
Another strategy for some rusts is eradication of the field corn varieties with some resistance to corn smut,
most commonly used sweet corns are susceptible. Highly

susceptible varieties are grown, however, and the galls,
which are edible before the teliospores mature, are sold in (a)
Mexican markets. This delicacy is known as huitlacoche.
Deuteromycota (mitosporic fungi)

Some fungal anamorphs have no known sexual state in
their life cycle and thus remain unclassified. These organ-
isms, also known as mitosporic fungi, are placed in an
artificial group called the Deuteromycota or the Fungi
Imperfecti. They reproduce mitotically via conidia and (b)
are considered anamorphic partners of teleomorphs in the
Ascomycota or Basidiomycota. Molecular techniques
(c)
have helped to place mitosporic fungi with their asso-
ciated teleomorphs.
Mitosporic fungi share many of the same features as
their sexual relatives. Most of the 17 000 described species (d)
are terrestrial and survive as saprophytes. Some trap Figure 2 Asexual reproduction in mitosporic fungi. (a) Single
nematodes, others are symbionts (lichens, grass endo- conidia varying in shape, color (hyaline or brown), and number of
phytes, mycorrhizae, and weak or primary pathogens). cells. (b–d) Asexual fruiting structures. (b) Conidia produced on a
distinct conidiophore. (c) Synnema. (d) Pycnidium. Reproduced
Mitosporic fungi are divided into three informal classes: from Shishkoff N, with permission of CRC Press.
Agonomycetes (mycelia sterilia, mentioned previously),
coelomycetes (spores are produced in fruiting structures
called conidiomata), and hyphomycetes (spores are pro- flecked pattern of necrosis appears on the leaves, which
duced on separate conidiophores). defoliate, reducing yield. Crop rotation, resistant vari-
Classification within these groups is based on conidio- eties, and fungicides are used to manage this disease.
phore, conidium, and hyphal characteristics. Conidia can
be one-celled, two-celled, or multicellular with transverse
or oblique septa. They have different shapes, appearing Diagnosis
filiform (thread-like), ovoid (egg-shaped), clavate (club-
shaped), cylindrical, stellate (star-like), or branched. They Classically, disease diagnosis, whether the disease is due
may be pigmented or have appendages. Conidia within to biotic or abiotic causes, is a process where symptoms
the hyphomycetes have distinct conidiophores and are (host response to disease agent) and signs (pathogen thal-
produced singly, as synnema (a group of conidiophores lus or reproductive structures) are interpreted. Many
are united together to form a stalk), or as sporodochia diseases caused by fungi can be easily identified due to
(clusters of conidiophores that form a cushion of hyphae highly characteristic symptoms or signs. For example,
on the host surface). Coelomycetes produce conidia in powdery mildew fungi are identified by the presence of
fruiting structures such as pycnidia (a flask-shaped struc- mycelia and spores (signs) on the surface of susceptible
ture lined on the inside with conidiophores) and acervuli plant tissue. Corn smut is diagnosed by the presence of
(conidiophores lay side by side in a flat, saucer-shaped teliospores (sign) in smutted galls (symptom). For many
configuration) embedded in host tissue (Figure 2). other diseases, however, the cause is not so obvious,
Alternaria solani is an example of a pathogenic hypho- especially in cases where damage due to the primary
mycete. The fungus causes tomato early blight, which is cause is so advanced that secondary fungi (decomposers
one of the most important tomato diseases in the United or opportunistic organisms) invade the dead or dying
States. The pathogen overwinters as chlamydospores in tissues.
plant debris, which serves as a source of inoculum in Some of the standard processes used in diagnosis
spring. Lesions develop on leaves, stems, and fruit; con- include microscopic examination of diseased tissues to
idia, formed on distinct conidiophores, are disseminated look for signs such as mycelium or fruiting structures.
by wind, rain, insects, farm machinery, and infected seeds. Samples of diseased tissues are often placed in a moist
Entire plants defoliate and die. chamber to encourage the growth of aerial mycelium and
Brown spot of soybean is caused by the coelomycete spores. A pathogen that is easily coaxed to produce myce-
Septoria glycines. The fungus overwinters as pycnidia lium in this matter is B. cinerea, a pathogen of aerial plant
embedded in the debris of stem and leaf tissues. During parts that produces light, easily dispersed conidia on long
warm, moist weather, the fungus grows as a saprophyte, conidiophores. Another standard technique is plating
eventually invading leaf tissues through the stomates. A small pieces of surface-disinfested symptomatic host
tissue on laboratory media. Fungal colonies that grow Unlike the earlier concept of disease control, where
from the tissue into the agar medium are examined for the goal was to eradicate all pathogen propagules within a
color, hyphal characteristics, and spores. A selective med- given crop system, the concept of disease management
ium designed to support the growth of certain pathogenic seeks to reduce pathogen populations to levels below a
organisms over saprophytes, such as PARP (pimaricin þ certain tolerable, often economic, threshold. Indeed, era-
ampicillin þ rifampicin þ pentachloronitrobenzene agar, dication of pests is seldom even possible. Management
a selective medium for pythiaceous fungi), is especially strategies are twofold: (1) to reduce initial pathogen
useful. inoculum through pathogen avoidance (geographical
Antibody-based tests, such as ELISA (enzyme-linked and temporal changes in planting), exclusion (use of
immunosorbent assay), are used to target certain patho- pathogen-free plants and plant parts), and quarantine;
gens, especially soil pathogens such as Pythium, and (2) to reduce existing inoculum through pathogen
Phytophthora, and Rhizoctonia. Commercially available test eradication and plant protection.
kits for many pathogens can be purchased by diagnostic For example, species of rubber (Hevea spp.) are native
laboratories, and some kits are available for use in the to the Amazon basin; latex was harvested from these trees
field. These tests can detect the targeted antigen and in the wild. When American automakers attempted to
whether the pathogen is living or not, and may take grow these trees in plantations to supply latex for tires
about 3 h to complete. in the 1930s, South American leaf blight, caused by the
Newer molecular diagnostic techniques include stan- ascomycete Microcyclus ulei, also native to the same region,
dard PCR and real-time PCR. In standard PCR, small bits destroyed the trees. Rubber plantations were moved to
of DNA from the target organism are extracted and the tropical areas of Southeast Asia to avoid the disease.
amplified many fold, and the DNA product is visualized Quarantines are needed to protect these trees from
as bands on a gel. This technique, although very sensitive, subsequent introduction to the pathogen. The first quar-
takes 12–24 h to complete and requires a steady hand to antine enacted in the United States, the Federal Plant
appropriately load the gels. In real-time PCR, DNA from Quarantine Act of 1912, was emended in 1915 to include
the target organism is also extracted, but the amplification chestnut blight, caused by the exotic pathogen
process is viewed as it happens. This technique, also very C. parasitica. In spite of the quarantine, chestnut blight
sensitive, is faster than standard PCR and obviates the went on to destroy the American chestnut throughout
necessity for gels. its range. The quarantine failed because the pathogen
Regardless of the detection technique, a good diagnos- was introduced to the Bronx Zoo in 1904, 10 years before
tician must take other factors into consideration. Most the quarantine was enacted. Seed certification programs
diseases are already described in the literature; identifica- are useful for diseases caused by fungi that infect the
tion of the host plant and knowledge of diseases that are embryo or infest the seed coat, such as Ustilago nuda, the
common to that host in a particular geographical area are cause of loose smut of barley.
very helpful. In addition, environmental conditions, such Once pathogens are introduced into a cropping sys-
as rainfall, temperature, soil conditions, and recent dis- tem, eradication of fungal propagules may be necessary to
turbances to the locale, help to pinpoint the causal agent. ensure sufficient yield. Chemicals, heat, cultural practices
A correct disease diagnosis is essential if the appropriate (fallow, flooding, rouging), and biological control are used
control measures are to be applied. to eradicate pathogens from growing areas, machinery,
soil, tools, and plant parts used in propagation (cuttings,
tubers, corms, and seeds). As mentioned previously, an
Control eradication program of the alternate host was used to help
control stem rust of wheat. This program was only mar-
Modern agriculture tips the ecosystem balance to favor ginally successful because fungal propagules for this
the host plant. As a result, growing conditions are artifi- disease (urediniospores) are blown to wheat-growing
cial: often, the same crops are grown yearly in the same areas from another location.
field, providing opportunities for pathogen populations to Biological control, where other microorganisms are
increase; crops are grown in monoculture, where all used to reduce pathogen inoculum, is useful in some
plants are genetically similar, thus all of them are suscep- situations where environmental conditions are suitable
tible to a virulent pathogen; environmental conditions, for development of the biological control agent. An exam-
especially water management, may be modified to ple of a successful biological control program was the use
enhance conditions favorable for pathogen development; of hypovirulence to manage chestnut blight in Europe.
and high fertilizer inputs may encourage the development Hypovirulent strains of C. parasitica contain a virus that
of succulent host tissues, which, in many cases, are more reduces the virulence of the fungal pathogen. Chestnut
susceptible to pathogen attack. trees affected with the hypovirulent strain do not develop
lethal cases of chestnut blight. The hyphae of mycelia See also: Fungicides and other Chemical Approaches for
within the same anastomosis group will fuse, passing the use in Plant Disease Control; Plant Pathogens:
virus from one mycelium to another. Hypovirulence has Oomycetes (water mold); Plant Pathogens and Disease:
worked well in Europe, where the few genetically differ- General Introduction; Plant Pathogens and Disease: Newly
ent strains there readily anastomose. This has not worked Emerging Diseases; Plant Disease Resistance: Breeding
in the United States, where there are many more genetic and Transgenic Approaches; Plant Disease Resistance:
strains of the pathogen. Natural, Non-Host Innate or Inducible; Stramenopiles
Practices that protect plants from pathogens include
the use of genetic resistance, cultural practices (barriers, Further Reading
mulches, plant nutrition, management of water and soil
Agrios GN (2005) Plant Pathology, 5th edn. Burlington, MA: Elsevier
edaphic factors), chemicals (fungicides), and biological Academic Press.
control. Often, these strategies are more cost-effective Barr DJS (2001) Chytridiomycota. In: McLaughlin DJ, McLaughlin EG,
than trying to eliminate pathogen propagules from a and Lemke PA (eds.) The Mycota VII. Part A: Systematics and
Evolution, pp. 93–112. Berlin: Springer-Verlag.
given production system. The apple scab pathogen, Braselton J (2001) Plasmodiophoromycota. In: McLaughlin DJ,
V. inaequalis, requires a period of leaf wetness for the McLaughlin EG, and Lemke PA (eds.) The Mycota VII. Part A:
spore penetration process to occur. Leaf material that is Systematics and Evolution, pp. 81–91. Berlin: Springer-Verlag.
Carlile MJ, Watkinson SC, and Gooday GW (2001) The Fungi, 2nd edn.
permitted to dry readily may be protected from the fungal San Diego: Academic Press.
pathogen. The use of seed treatment and protectant fun- Deacon J (2006) Fungal Biology, 4th edn. Malden, MA: Blackwell
gicides is essential for the management of many fungal Publishing.
Hawksworth DL, Sutton BC, and Ainsworth GC (eds.) (1983) Ainsworth
diseases, especially for those where the crop value is high & Bisby’s Dictionary of the Fungi. Kew Surrey: Commonwealth
and disease management is essential for adequate yield. Mycological Institute.
Schumann GL (1991) Plant Diseases: Their Biology and Social Impact.
For example, susceptible apple tissue is sprayed on a St. Paul, MN: APS Press.
protectant basis for apple scab control. In certain locations Schumann GL and D’Arcy CJ (2006) Essential Plant Pathology. St. Paul,
of the United States, susceptible Douglas-fir in Christmas MN: APS Press.
Sinclair WA and Lyon HH (2005) Diseases of Trees and Shrubs, 2nd
tree plantations are sprayed with fungicide in the spring edn. Ithaca: Cornell University Press.
to control Rhabodocline needle cast; this disease makes Taylor JW, Spatafora J, and Berbee M Ascomycota. Tree of Life Web
trees unfit for sale where it occurs. Perhaps the wisest Project. http://tolweb.org.
Trigiano RJ, Windham MT, and Windham AS (eds.) (2008) Plant
management strategy is the use of resistant plant material. Pathology Concepts and Laboratory Exercises, 2nd edn. Boca
For example, since fungicide use is cost-prohibitive for Raton: CRC Press.
use in many cereals, genetic resistance is necessary for Ulloa M and Hanlin RT (2000) Illustrated Dictionary of Mycology. St.
Paul, MN: APS Press.
management of stem rust of wheat. Classic plant breeding Volk TJ (2001) Fungi. In: Encyclopedia of Biodiversity, Vol. 3,
programs as well as biotechnology and genetic engineer- pp. 141–163. New York: Academic Press.
Volk TJ Tom Volk’s Fungi. Department of Biology, University of
ing have been used to develop germplasm that is resistant Wisconsin-LaCrosse. http://botit.botany.wisc.edu.
to fungal pathogens of major economic and social Webster J and Weber RWS (2007) Introduction to Fungi, 3rd edn.
importance. Cambridge: The University Press.

Fungi - Plant Pathogenic

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Fungi: Plant Pathogenic

A B Gould, Rutgers University, New Brunswick, NJ, USA

Defining Statement Pathogen Groups

Glossary hyperplasia Excessive cell division.

Defining Statement Introduction

Protozoa Stramenopila Eumycota

Plasmodiophoromycota Oomycota Chytridiomycota Zygomycota Ascomycota Basidiomycota

Habitat Aquatic Aquatic Aquatic/ Terrestrial Terrestrial Terrestrial

Nutrition host. Biotrophs, which are highly host specific, do not

Table 2 Function of digestive exoenzymes produced by pathogenic fungi

Enzyme Substrate Function

Table 3 Plant pathogenic oomycetes

Order Characteristics Examples

Saprolegniales Well-developed mycelium, zoosporangium is long and Aphanomyces euteiches (pea)

Albuginaceae (white Sporangia formed in chains on club-shaped Albugo candida (crucifers)

Table 4 Plant pathogenic ascomycetes

Class Ascoma Examples

Archiascomycetes Naked Taphrina caerulescens (oak leaf blister)

Table 5 Plant pathogenic basidiomycetes

Order Characteristics Examples

Table 6 Spore stages associated with rust fungi

0 Pycnium (spermagonium) Pycniospore (n) Primary Alternate, or aecial

most commonly used sweet corns are susceptible. Highly

Deuteromycota (mitosporic fungi)

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