Diseases of Rice
Diseases of Rice
Diseases of Rice
DISEASES OF RICE
Rice Blast
Blast disease of rice is noticed in all rice growing areas and is considered the most serious
disease because of its devastating nature, widespread distribution and existence of several
physiological races of the causal organism. This disease was reported in rice seedlings as early as
1637 from China. At that time, heat was ascribed to be the cause of the disease and was hence
called ‘rice fever disease”. Metcalf (1907) from South Carolina, USA was the first to name this
disease ‘blast’. According to Padmanabhan (1965) blast was first recorded in India when a
devastating epidemic occurred in 1913 in the Tanjore delta area of erstwhile Madras State.
Maximum losses due to blast occur when seedlings or plants are killed at tillering stage or when
panicles are infected. In India, 75% loss of grain occurred in 1950 in susceptible cultivars while,
in general, 40-50% yield loss has been reported due to epidemics in different rice growing areas of
the world. The time of infection, part of the plant infected, variety of plant, environmental factors
etc. influence disease severity and eventually yield loss.
Symptoms
The disease sets in at any growth stage of rice plant although it is more frequent in the
nursery and flowering stages. Symptoms are noticed generally on leaves, nodes, panicles and
grains, and rarely on leaf sheath.
Symptoms on leaf known as ‘leaf blast’ appear as minute brown specks at first, which
enlarge to form spindle shaped lesions with pointed ends in susceptible cultivars. The centre of
the spot is grey or white with brown on reddish brown margin and with conspicuous yellow halo
around the spots. Heavily infected leaves soon become blighted and die. Severe infection in the
nursery stage causes death of seedlings and is known as ‘Nursery blast’. Badly affected nurseries
can be recognized from a distance by their brownish, scorched appearance.
Infection can also occur on nodes. The sheath pulvillus rots and turns black which
separates from the leaf on drying. The connected node also turns black and rots which leads to
death of the parts above the node. This is the ‘node blast’ phase of the disease.
The most destructive phase of the disease is when the pathogen infects panicles. This
stage is called ‘neck rot’, ‘neck blast’, ‘broken neck’ or ‘panicle blast’. The symptom is
characterized by the appearance of black or greyish brown lesions at the base of the panicle
(neck). The infected tissue shrivels and turns black. This weakens the tissue and panicles often
break at the neck region and hang down. If the symptoms occur at the time of panicle emergence,
entire grains of the panicle turn chaffy. The attack at later stages causes partly filled grains.
Brown to black spots are also sometimes seen on the infected glumes.
Pathogen
Bipolaris oryzae (Syn. Drechslera oryzae) - Anamorph
Cochliobolus miyabeanus - Teleomorph
The anamorph stage of the fungus produces fluffy, aerial, dark greenish gray cottony
mycelium. The conidiophores are solitary or in small groups and olivaceous in colour. Conidia
are olivaceous brown, usually curved, broader at the middle or just above the middle and taper to
rounded ends. The teleomorph produces perithecia that are globose to depressed, globose, with
the outer wall dark yellowish brown and pseudoparenchymatous, The asci are cylindrical to
fusiform. The ascospores are filamentous or long cylindrical, hyaline or pale olive green and
coiled together.
Epidemiology
The fungus survives in the seed and infected plant parts. The mycelium is present on seed
coat, pericarp, and glumes and sometimes in the endosperm. Infected grains show brown stains
and have large number of conidiophores and conidia. Apparently healthy grains from infected
plants also carry inoculum. The fungus can survive in seed for more than 4 years. However,
under warm humid conditions, the conidia cannot live long, yet it can carry the pathogen from one
cropping season to the next. Seed thus serves as the primary source of inoculum. Germination of
the infected seeds is reduced to 10-30%. Seedlings developing from such seeds may develop root
and shoot decay and die before maturity.
The pathogen also survives as mycelium and conidia in infected crop debris in soil.
Volunteer rice plants and graminaceous weeds in the rice bunds also help in survival of the
fungus.
The conidia produced on the infected plant parts are carried by wind and contribute to the
secondary spread of the disease.
Temperature of 25-30o C is optimum for disease development. RH of more than 89% is
required for successful infection. However, dry soil conditions predispose plants to disease more
than wet soil. High atmospheric humidity and low soil moisture inhibit the absorption of Si and K
and reduce the SiO2 and K2O content of leaves, and hence increase the susceptibility of brown
spot. Shaded conditions enhance disease development.
Management
1. Use certified and healthy seed
2. Use resistant varieties such as Kanchana, Remya, IR-36 etc
3. Seed treatment is effective in managing seedling blight. This can be done with
carbendazim (1g/kg seed), copper oxychloride (2-4g/kg seed) or mancozeb (2-4g/kg
seed). Treating the seeds in hot water at 52°C for 10 minutes is also effective. Seed
infection in the field can be reduced by spraying fentin acetate and mancozeb in the
proportion 1:5 at 0.2% on the crop at heading and grain maturation stage.
4. Poor nutrient status of the soil can be corrected by balanced application of fertilizers,
good water management etc.
5. Application of soil amendments such as neem cake 10 T/ ha can help in reducing
disease severity.
Narrow brown leaf spot
Symptoms
The disease affects all the aerial parts of the plants. Short, linear brown lesions are noticed
on leaves, leaf sheaths, glumes etc. Size and colour of the lesion depend on the resistance and
susceptibility of the cultivars.
Pathogen
Cercospora janseana (Syn.Cercospora oryzae) - Anamorph
Sphaerulina oryzina - Teleomorph
The conidia are cylindrical to clavate or catenulate, 3-10 septate.
The teleomorph produces perithecia, which are scattered or gregarious, globose or subglobose and
black in colour. Asci are cylindrical or club-shaped, round at the top. Ascospores are spindle
shaped, straight or slightly curved 3 septate hyaline.
Epidemiology
The pathogen carried externally on seed serves as the primary inoculum. The wind borne
conidia produced from lesions act as secondary inoculum and help to spread the disease in a
standing crop.
Management
Same as in Brown leaf spot.
Leaf scald
Symptoms
Leaf scald is seen on mature leaves as zonate lesions, which start as diamond shaped water
soaked areas at the tips and extend downwards. In the affected areas, dark brown serrate or
dentate markings are visible. Varieties with broad leaves are more susceptible than those with
narrow leaves.
Pathogen
Microdochium oryzae (Syn. Rhynchosporium oryzae) - Anamorph
Monographiella albescens - Teleomorph
The fungus produces white to buff coloured growth. Mycelium is septate, branched and
light yellow in colour. Conidia are sickle shaped, single celled when young, and two-celled when
mature, occasionally 2-3 septate.
Epidemiology
The pathogen is externally and occasionally internally seed borne. Infected seeds on
germination produce disease in seedlings.
Management
Fungicidal seed treatment with mancozeb (0.15%) + benomyl (0.15%) has been found
effective in eradicating the pathogen from seed.