African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 1, 1996, pp. 111-113

Short communication: Survey of leaf spot-causing microorganisms on yams

A.N. AMUSA, T. IKOTUN and J.O. BANKOLE

Phytopathology Laboratory, Department of Agricultural Biology, University of Ibadan, Ibadan, Nigeria

(Received 10 August 1994; accepted 28 October 1995)

Code Number: CS96047
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ABSTRACT

A survey of leaf-spot pathogens of cultivated Dioscorea alata and D. rotundata in Nigeria was conducted. Colletotrichum gloeosporioide was the predominant organism judging by its frequencies of occurrence of 45.6% and 42.7 % in 1992 and 1993, respectively. Other fungi found associated with yam leaf-spot included Curvuloria eragrostides, Pestalotia macrotrichia, Sclerotium rolfsii, Rhizoctonia solani, Colletotrichum graminicola, C. lindemuthianum, Botryodiplodia theobromae, Curvuleria pallanscen, Fusarium oxysporum and Macrophorria sp. In pathogenicity test, however, only C. glocosporioides, C. graminicola, S. rolfsii, R. solani, eragrostides and P. macrotrichum were able to induce necrotic lesions on leaves of both D. alata and D. rotundata, suggesting that only these organisms were the primary leaf-spot pathogens of yams.

Key Words: Diascorea alata, D. rotundata, fungi, pathogenicity

RESUME

Une enquete sur les pathogenes de taches foliaires d'ignames cultives, le Dioscorea alata et D. rotundata, etait effectuee au Nigeria. Le Calletotrichum glaeasporioride etait l'organisme predominant de part sa frequence d'apparition de 45,6% et 42,7% en 1992 et 1993, respectivement. Les autres champignons associes au taches foliaires d'ignames etaient notamment Curvuloria eragrostides, Pestalatia macrotrichia, Scleratium rolfsii, Rhizoctania solani, Colletotrichum graminicola, C lindemuthianum, Batryadiplodia theobromae, Curvuleria pallanscen, Fusarium oxysparum et Macrophorria sp. Cependant, au point de vue de test de pathogenecite, il n'y avait que C glacosporioides, C graminicala, S. rolfsii, R. solani, eragrostides et P. macratrichum qui etaient capable d'induire des lesions necrotiques sur les feuilles de D. alata et D. rotundata; ce qui suggere que seuls ces organismes etaient les pathogenes primaires de taches foliaires d'ignames.

Mots Cles: Dioscorea alata, D. rotundata, champignon, pathogenecite

INTRODUCTION

Yams belong to the family Dioscoreceae and to the genus Dioscorea. They are staple food crops in tropical and subtropical Africa, Central and South America, parts of Asia, the Caribbean and Pacific Islands (Coursey, 1967; Adelusi and Lawanson, 1987). World production of yam in 1988 was 23.7 million tonnes, of which about 95% was grown in Africa (FAO, 1989). Nigeria produces 71.2% of the total world production, with yam tubers being grown mostly in Bendel, Ondo, Anambra, Lagos, Ogun, Oyo, Benue, Plateau, Kogi and Niger States.

Since yams grow for five to ten months in the field, the shoots, roots and tubers are exposed to attack by various disease causing organism. While the diseases of yam tubers have been well investigated and documented (Okafor, 1966; Adeniji, 1970; Ikotun, 1989), the leaf spot disease causing fungi have not been given a similar attention, and the only records of leaf spot diseases of cultivated yams are those of Bailey (1966) and Emua and Fajola (1980). This study was, therefore, designed to survey the incidence of leaf spot- causing organisms on yams in Nigeria, and to ascertain their pathogenicity.

MATERIALS AND METHODS

The survey and sampling of leaves of two yam species (D. alata and D. rotundata) with necrotic spots was carried out during the growing seasons of 1992 and 1993, in farmers fields in the western, middle belt and eastern zones. These zones comprise the major yam producing states in Nigeria, namely Oyo, Ogun, Kwara, Niger, Kaduna, Benue, Imo, Kogi, Enugu, Anambra, Abia and Cross-River State. A total of 250 samples were collected and evaluated.

Isolation. Sample leaves with leaf spot symptoms were washed in running tap water, cut into small pieces, surface sterilised using 10% NaOCl3, and rinsed in four successive changes of sterile distilled water. The infected portions were then plated out in Potato Dextrose Agar (PDA) and incubated for 6 days at 26 C under 12 hr of alternating light and darkness. The causative pathogens were identified using a compound microscope and by comparing them with standards obtained from the Advanced Pathology Laboratory at the International Institute of Tropical Agriculture, Ibadan, Nigeria.

Pathogenicity of the isolated fungi. Spore suspensions of the fungi were prepared by washing the surface of the cultures with sterile distilled water and passing the suspension through a four layer Muslim cloth to remove fungal mycelia and other debris. Estimates of spore concentration were made on a heamocytometer slide, and the suspensions adjusted for all the fungi to 1.6 x 10^-1 spores per ml of sterile distilled water. These were used singly or together to spray the leaves of 10 weeks-old D. alata and D. rotundata raised on steam sterilized soil from yam. In the case of Sclerotium (Corticium) rolfsii and Rhizoctonia solani, a single sclerotium of each of these fungi was used to inoculate leaves of the test plants.

All treated yam plants in quadruplicates were incubated at high relative humidity for 24 hr before they were left for observation on the development of disease symptoms.

RESULTS AND DISCUSSION

Colletotrichum gloeosporioides (causing yam anthracnose) was the most encountered in this study, with frequencies of 45.6 and 32.7% in 1992 and 1993, respectively (Table 1). This was followed by Curvularia eragrostides, which caused concentric leaf spots, and P. macrotrichia and Sclerotium rolfsii associated with zonate leaf spot with 16.6%, 8.3% and 7.5% incidence in 1992 and 15.4%, 7.5% and 6.6% in 1993. Colletotrichum graminicola and R. solani occurred at frequencies of between 0.8 and 2.1% in the two years of survey. Others were F. oxysporum, C. lindemuthianum, B. theobromae and C truncatum. The least encountered was Macrophoma sp. (Table 1).

Two of the fungi, C. gloeosporioides and Fusarium oxysporium were found with a leaf spot at a frequency of 1.3% and 2.1% in 1992 and 1993, respectively, while C. gloeosporioides and P. macrotrichia were both isolated from the same necrotic spots with frequencies of 1.3% and 3.2% in 1992 and 1993, respectively. Other fungi were found with varying frequencies. (Table 1).

C. gloeosporioides and S. rolfsii were mostly encountered in the southern part of Nigeria, while P. macrotrichia and R. solani were also widely found in the middle belt zone. C. truncatum and C. lindemuthianum were isolated from leaves of Dioscorea sp. obtained from fields where yams were intercropped with cowpea, C. graminicola and Curvuloria pallenscens were isolated from plots where yams were intercropped with cereals.

In pathogenicity tests, C. gloesporioides, C. graminicola, C. eragrostides, S. rolfsii, P. macrocrichia and R. solani were able to induce necrotic lesions on yam leaves. These were considered primary pathogens of the crop graminicola which are known to be pathogenic to grasses (CIMMYT, 1983).

There was no concrete evidence to suggest that the occurrence of these pathogens in association with leaf spots was due to the effect of intercropping with cowpea, maize or sorghum. These fungi may be trasciently resident on these hosts or are otherwise over-wintering in the necrotic lesions already induced by C. gloesporioides. Colletotrichum sp. are known to be ubiquitous plant pathogens which are non-host specific; it is, therefore, not surprising that C. graminicola could induce necrotic spots on yam leaves. Because Curvularia sp. The survival or presence on necrotic tissues or lesions on yam leaves by Curvularia sp. was not surprising, because they are are nectrotrophs just like the Colletotrichum sp..

More than one pathogen species were found associated with a necrotic lesion on yam leaves. The effect on the leaves of the combined infections might be responsible for the rapid devastation of yam leaves that are normally observed in the field.

TABLE 1. Occurrence of leaf spot- causing fungi on yam in Nigeria

Fungi                        Frequency of occurrence (%)
                                   1992   1993
                               --------------------------
Colletorichum gloeosporioides     45.6    32.7
Sclerotium rolfsii                 7.5    6.6 
C. lindemuthianum                  2.1    4.2 
Pestalotia macrotrichia            8.3    7.5 
Botryodiplodia theobromae          5.0    4.2 
Curvuleria pallenscens             2.1    2.1 
Curvuleria eragrostide            16.6   15.4
C. gramicola                       2.1    1.3 
Fusarium oxysporium                4.2    5.4 
Rhizoctonia solani                 0.8    1.3 
C. truncatum                       0.0    0.4 
C. gloeosponoides/                 
Pestalotia macrotrichia            1.3    3.2 
C. gloeosporioides/R. solani       0.4    0.4 
C. gloeosporioides/                 
Curvuleria eragrostides            0.8    1.7  
C. gloeosporoides/                 
C. graminicola                     1.7    0.4
C. gloeosporiodes/                 
Fusarium oxysporium                1.3    2.1

REFERENCES

Adelusi, A.A. and Lawanson, A.O. 1987. Disease induced changes in carotenoid content of edible yam (Dioscorea spp.) infected by Botryodiplodia theobrontae and Aspergillus niger. Mycopathologia 98:49-58.

Adeniji, M.O. 1970. Fungi associated with storage decay of yam in Nigeria. Phytopathology 60: 590-592.

Bailey, A.G. 1966. A Check List of Plant Diseases in Nigeria. Federal Department of Agricultural Research, Ibadan, Memo 96. 42 pp.

Baker, K.F. and Cook R.J. 1974. Biological Control of Plant Pathogens. Freeman and Company. 433 pp.

CIMMYT, 1983. Common Diseases of Small Grain Cereals. A Guide to Identification, Zillinsky, F.J. (Ed.), 141pp. CIMMYT.

Mexico.

Coursey, D.G. 1967. Yam storage 1: A review of storage practices and information on storage losses. Journal of Stored Products Research 2:229-244.

Emua, S.A. and Fajola, A.O. 1980. Leaf spot diseases of cultivated yam (Diascorea sp.)in South Western Nigeria. Journal of Root Crops Vol. 6 No. 1812, 37 pp.

FAO, 1989. Production Year Book. FAO. Rome, Italy.

Ikotun, T. 1989. Diseases of yam tubers. International Journal of Tropical Plants Diseases 7:1-21.

Okafor, N. 1966. Microbial rotting of stored yam (Dioscorea spp.) in Nigeria. Experimental Agriculture 2:179-182.

Copyright 1996 The African Crop Science Society