Vol. 10(50), pp.

4538-4542, 10 December, 2015

DOI: 10.5897/AJAR2015.10272
Article Number: 1E3ACFF56305
ISSN 1991-637X
African Journal of Agricultural
Copyright ©2015 Research
Author(s) retain the copyright of this article
http://www.academicjournals.org/AJAR

Full Length Research Paper

Isolation and identification of some pathogenic fungi

associated with cassava (Manihot esculenta Crantz)
root rot disease in Cameroon
Nyaka Ngobisa A. I. C1*, Kammegne Djidjou P.2, Ntsomboh Ntsefong Godswill3, Mbenoun M.4,
Zok Simon1 and Fontem Dominic5
1
Institute of Agricultural Research for Development (IRAD), Ekona Regional Centre, PMB 25 Buea, Cameroon.
2
Department of Biochemistry, Faculty of Sciences, University of Douala, Cameroon.
3
Phytopathology Unit, IRAD-CEREPAH La Dibamba, B. P. 243 Douala, Cameroon.
4
Institute of Agricultural Research for Development (IRAD), Nkolbisson Regional Centre, PMB 2067,
Yaoundé, Cameroon.
5
Department of Plant Protection, Faculty of Agriculture, University of Dschang, Cameroon.
Received 11 August, 2015; Accepted 29 October, 2015

Root rot diseases constitute a major constraint to cassava production in Cameroon. However, not much
is known about the identity of pathogens associated with them. It is in this light that this study was
realized with the aim of characterizing the various root rot diseases and identifying their associated
fungal causal agents in Cameroon. Sixty four cassava stems with root rot symptoms were sampled in
the Littoral, Southwest and West Regions of Cameroon. Results revealed that cassava root rot (CRR) is
either wet (soft) or dry depending on the region of study. Isolation was done on PDA medium enriched
with Chloramphenicol. After purifying thrice on the PDA medium, 20 isolates were collected.
Identification with the help of the Barnett and Hunter key revealed the existence of seven fungi
including Colletotrichum sp., Fusarium sp., Pestalotia sp., Geotrichum sp., Sphaerostilberepens,
Trichoderma viride and Botryodiplodia theobromae.

Key words: Cassava root rot, pathogen, sustainable agriculture, disease control.

INTRODUCTION

The actual crop yield as percentage of potential yield is (FAO, 2012; Valipour, 2014; Valipour et al., 2015). One of
more than 60% for North America, Western and Central the world’s most important food crops is cassava
Europe, but is less than 50% for South America and (Manihot esculenta Crantz) which belongs to the family
North Africa and it is about 30% for Central America and Euphorbiaceae. Cassava roots and leaves serve as an
the Caribbean, Eastern Europe and sub-Saharan Africa essential source of calories and income throughout the

*Corresponding author. E-mail: boulnyaka@yahoo.com.

Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution
License 4.0 International License
 Ngobisa et al. 4539

tropics. Most people in Africa, Asia and Latin America regions and carried to the Crop Protection Laboratory of the
depend on the cassava crop for their food and income. A Department of Plant Protection, Faculty of Agriculture, University of
Dschang where fungal pathogens were isolated.
significant progress has been observed in cassava
production in recent times. In 2005, cassava occupied the
5th rank in the world production of food crops after maize, Collection of samples
rice, wheat and solanum potatoes. World production of
fresh cassava tubers increased from 189,099,633 tons to Two farmers’ farms per locality were surveyed for the presence of
232 million tons in 2008 with Sub-Saharan Africa alone plants with external symptoms such as leaf browning and
discoloration of the lower part of the stems, and generalized wilting,
producing 118 million tons per annum constituting about which are most often indicative of root infections. Of all the plants
half of the world’s production (FAOSTAT, 2010). In identified, four were off-rooted from each farm and observations
Cameroon where roots and tubers account for 70% of the were done on the phytosanitary status of their root system.
total cultivated area and 46% of food crop production, Samples of partially rotten tubers were taken from these infected
total cassava production was estimated at 2,882,734 tons plants and carried to the laboratory where fungal isolations were
done using potato dextrose agar (PDA) medium.
in 2008 (Agristat, 2009). In Cameroon, 80% of urban
households consume cassava products on a daily basis,
and about 90% of small-scale producers market at least a Isolation and purification of fungi
small part of the cassava they produce.
Despite the relative progress observed in world annual Fungal isolation was done on PDA enriched with chloramphenicol
yield of cassava, its cultivation is faced with several pests (150 ppm) in order to avoid bacterial growth. The culture medium
was poured in Petri dishes and allowed to solidify. Tubers were
and diseases. The fungal root rot disease of cassava washed with tap water. With the help of a sterilized scalpel, 1 cm
which affects the tubers, is a disease caused by infection fragments were collected from the necrotic front and disinfected
of the roots by fungi found in humid or poorly drained soil with 95°C alcohol with the help of a sterile pincer. The fragments
(Silvestre and Arrandeau, 1983). It is characterized by were then washed with distilled water and dried with sterile blotting-
browning and wilting of leaves, accompanied by loss of paper. This exercise was undertaken under a laminar flow hood in
water which may eventually lead to the death of the plant. the presence of a Bunsen burner in order to assure aseptic
conditions. These sterilized explants were cultured on PDA-
The other symptoms are swelling of roots and a light chloramphenicol medium at 25°C in the dark. After ten days of
brown coloration observed when the roots split in the soil incubation, fungal colonies which emerged from the explants were
or when they are cut open. In Africa, CRR causes sub cultured individually on new PDA simple culture medium. This
enormous yield losses. It actually hinders the synthesis action was repeated thrice until pure cultures were obtained. A
and storage of nutrients in the roots. This consequently collection of 20 isolates was constituted. Microscopic observation of
each isolate was done with an optical microscope (model Olympus
limits plant development, reduces number of roots and
BH-2) at a magnification of 400x, and fungal identification was done
their ability to form tubers and become mature, hence with reference to the key of Barnett and Hunter (1972).
limiting production (Msikita et al., 2000).
In Cameroon, about 36% of farmers classify CRR as
the second cause of reduced yields in the cassava
RESULTS
sector. However, proper identification of pathogenic fungi
associated with this disease is yet to be done (Messiga et
Description of root rot symptoms
al., 2004). In order to address this situation, this study
was realized in the Littoral, South-west and West
At the first site, a swelling of the tuber and browning of
Regions of Cameroon with the aim of identifying the
the bark close to the stem were noted on the roots
pathogenic agents associated with CRR through their
collected from all localities. Splits of these roots showed
isolation from infected plants and morphological
light brown coloration which is characteristic of CRR di-
description of their fructifications.
sease. A pungent smell was also noted. It was observed
that the rotting was different for samples collected from
MATERIALS AND METHODS the different localities (Table 1). In fact, rotting of some
roots was humid and pasty which is characteristic of soft
Study sites root rot while on others it was friable, characteristic of dry
rot. It has been reported that relative humidity plays a
Visits and observations were done between March and June 2010 major role in cassava fungal disease development
in cassava farms in eight localities, namely Douala, Dibombari, (Makambila, 1994).
Souza, Mondoni, Batoke, Ekona, Kumba and Dschang, which are
found in the Littoral, West and Southwest Cameroun. These three
main agro-ecological zones are constituted principally of the humid Isolation and identification of pathogenic fungi
forest with monomodal rainfall for the Littoral and South-West,
characterized by an average temperature of 25°C, 4000 mm of
Fungal isolations realized on rotten cassava samples
annual rainfall with ferralitic sandy or sandy clay soil. The western
highland zone on its part has an average temperature of 20°C and from the eight localities revealed a great diversity of
about 1500-2600 mm rainfall with reddish ferralitic soil formed on species associated with cassava root rot. In all, 20 fungal
basalt. Samples were collected from infected cassava plants in these isolates were grouped into seven genera. Figure 1 presents
4540 Afr. J. Agric. Res.

Table 1. Typological characterization of cassava root rot.

Nature of Infected part

Region Ecosystem
infection Aerial part Root
Brown necrotic lesions, pasty Humid forest zone
Soft (wet) rot Wilting, leaves fall-off Littoral
rotting with monomodal rainfall
Brown necrotic lesions, pasty Humid forest zone
Soft (wet) rot Wilting, no falling of leaves Southwest
rotting with monomodal rainfall
Dehydration of plant, dark
Dry rot Friable rotting West Upper plateaux
brown coloration

Botryodiplodia theobromae Colletotrichum sp. Fusarium sp.

Pestalotia sp. Sphaerostilbe repens Trichoderma viride Geotrichum sp.

Figure 1. Diversity of fungal species associated with CRR as observed in PDA after 7 days.

presents the mycelial growth and coloration of colonies of mechanisms is of prime importance. This could be
each of these genera. achieved by isolation and identification of disease causal
The morpho-cultural and microscopic characteristics of agents on appropriate culture media. The choice of PDA
CRR causal agents isolated in this study are presented in medium for the isolation of the pathogenic fungal species
Table 2. in this study was based on its successful use in previous
With respect to localization, Fusarium sp. was the most studies (Maheshwari et al., 1999; Attrassi et al., 2005),
common genus with a 50% relative prevalence, followed which have been mentioned as an appropriate medium
by Botryodiplodia theobromae (15%), Colletotrichum sp. for isolation of a wide range of fungi. Isolation and
(10%) and Trichoderma viride (10%) (Table 3). identification of the pathogenic fungi in this study help to
show the presence of seven fungi, namely Colletotrichum
sp., Fusarium sp., Pestalotia sp., Geotrichum sp.,
DISCUSSION Sphaerostilbe repens, Trichoderma viride and
Botryodiplodia theobromae. With a relative prevalence in
Determination of infectious routes of plant pathogens and terms of isolation frequency equal to 50%, fungi of the
their mechanisms of infection are of great importance in genus Fusarium sp. were the most encountered. They
any disease control program (Twumasi et al., 2014). were respectively followed by B. theobromae (15%), T.
Host-plant resistance and biological control are the viride and Colletotrichum sp., respectively 10%, S.
cornerstones of crop protection measures against biotic repens, Geotrichum sp. and Pestalotia sp., respectively
stress on cassava (Herren, 1994). In fact, knowledge on 5%. With reference to the symptoms observed on the
the identity of the pathogen and disease infection roots/tubers of cassava, the genera Botryodiplodia sp. and
 Ngobisa et al. 4541

Table 2. Morpho-cultural and microscopic characteristics of fungi isolated in this study.

Fungi Morpho-cultural characteristics on PDA Microscopic characteristics

Very rapid growth, abundant cotton-like mycelium,
Botryodiplodia
colorless when young and becomes deep gray or Bicellular and ovoid conidia
theobromae
black with age
White cotton-like mycelium which turns to gray with
Colletotrichum sp. Abundant unicellular, fusiform conidia
time forming acervuli
White mycelium which turns to ochre-yellow on old Spores with crescent form. Septate
Fusarium sp.
cultures hypha.
Hyphae are septate. Spores are
Pestalotia sp. White cotton-like mycelium-forming black acervuli
fusiform.
Localized growth; colonies with twisted outlines,
They bear stilbospores. The mycelium is
Sphaerostilbe repens whitish when young and turning progressively red with
undifferentiated.
age
Rapid growth, sparse mycelium initially colorless, but
Trichoderma viride Pyramidal branched conidiophores.
rapidly turns green with profused green conidia
Hyaline Conidia. Septate hyphae with
Geotrichum sp. Less abundant milky white cotton-like mycelium
dichotomous branches.

Table 3. Relative prevalence of each pathogenic fungal species with respect to localization.

Fungi Localization Relative prevalence (%)

B. theobromae Douala, Batoke, Dschang 15
Colletotrichum sp. Batoke, Kumba, Dibombari, Dschang 10
Fusarium sp. Mondoni, Kumba, Souza, Douala 50
Pestalotia sp. Douala 5
Geotrichum sp. Kumba 5
S. repens Souza 5
Trichoderma viride Douala, Ekona, Kumba, Souza, Dschang 10

Sphaerostilbe sp. seem to be associated with dry root rot deterioration of tuberized roots of cassava (Noon and
while the genus Fusarium sp. is associated with wet (soft) Booth, 1977; Raimbault et al., 1985; Oyewole and
cassava root rot (Theberge, 1985). These symptoms cor- Odunfa, 1988). However, despite the fact that it
respond to a particular climatic condition. For instance, contributes to crop devastation, a study revealed that
the high prevalence of Fusarium sp. among our isolates Geotrichum sp. possibly produces dihydroisocoumarins
could be due to the conditions of high rainfall, particularly which could be capable of inhibiting the action of
common in the Littoral and Southwest regions, rendering Plasmodium falciparum (Palangpon et al., 2003).
the soils humid and favoring soft or wet rot. Fungi of the genus Trichoderma sp. are saprophytes
Damages caused by Colletotrichum sp. on cassava found in the soil. Their capacity to inhibit mycelial growth
have been signaled elsewhere. A special form of this of other fungi such as B. theobromae and Fusarium sp.
genus, Colletotrichum gloesporioides f. sp. Manihotis is has been put to evidence (Manjula et al., 2005). The
known to cause anthracnose diseases on cassava species viride though with very slow mycelial growth is
(Amusa, 1998; Magdalena et al., 2012) which is an im- seemingly capable of inhibiting the development of fungi
portant disease of cassava in tropical Africa, transmitted at a distance (Cherif and Benhamou, 1990). In fact,
through breeder seeds and post-harvest debris in the trichodermine (an antibiotic) has been derived from T.
field (Fokunang et al., 1997, 2001). The disease has viride (Dennis and Webster, 1971). The presence of
been reported to cause total crop failure where infected Colletotrichum sp. and Pestalotia sp. at the level of
propagation materials are used as seed sources (Ikotun cassava tuberized roots need further investigation given
and Hahn, 1991; Magdalena et al., 2012). that Makambila (1994) highlighted the responsibility of C.
The fungi of the genus Geotrichum sp. probably play a gloeosporioides in anthracnose of cassava stem. Their
role in the process of fermentation and post-harvest specific identification would permit establishment of the
4542 Afr. J. Agric. Res.

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