CHARACTERIZATION OF COLLETOTRICHUM GLOEOSPORJOIDES FROM YAM(DIOSCOREA SPP.) IN NIGERIA CHARACTERIZATION OF COLLETOTRICHUM GLOEOSPORIOIDES FROM YAM (DIOSCOREA SPP.) IN NIGERIA
KR GREENl, MM ABANG2 & C ILOBA2
lPiant Health Management Division, International Institute of Tropical Agriculture, clo Lambourn & Co. Ltd, Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, UK; 2Department of Crop Science, University of Nigeria, Nsukka, Nigeria.
Background and objectives
Foliar anthracnose caused by the fungus Colletotrichum gloeosporioides is a major constraint to the production of yam (Dioscorea spp.), with D. alata, the most widely distributed species being particularly susceptible to the disease . In Nigeria, the world's main producer of yam, the disease is particularly prevalent in the humid forest zone of the country . The development and utilization of anthracnose-resistant varieties represents a potential control measure that could form an important component of disease management strategies appropriate for resource-poor farmers. Reliable screening for sources of host-plant resistance among local landraces or new hybrids is, however, hampered by inadequate knowledge regarding variability within the pathogen population. The aim of this study was to characterize isolates of C. gloeosporioides collected from yam-based systems in three agro-ecological zones of Nigeria, according to their morphological, cultural and pathological variation. The disease responses of a range of yam cultivars (D. alata) were also determined using representative isolates, in order to provide a basis for the selection of isolates for use in routine screening for anthracnose resistance.
Results and conclusions
Fifty isolates of C. gloeosporioides from yam were separated by cluster analysis into six distinct groups on the basis of cultural characteristics, conidial and appressorial morphology, and production of setae and the ascogenous stage. A detached leaf assay used to determine the virulence of the isolates on D. alata cv. TDa 85100236 showed that the morphological groups also represented distinct virulence phenotypes. This result was supported by inoculation of detached leaves of five yam cultivars and of whole plants of seven yam cultivars with isolates representative of each group. In both cases, there were significant effects in disease severity due to cultivar, isolate and a highly significant cultivar x isolate interaction , stressing the necessity for careful isolate selection in the development of a routine method for screening yam germplasm for anthracnose resistance. There was clear evidence that morphological characteristics, particularly colony colour in combination with growth rate, can give an indication as to the virulence of isolates of C. gloeosporioides on yam. Results obtained will make a valuable contribution to future studies on genetic variability and dynamics within populations of C. gloeosporioides from Dioscorea spp.
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2. Green KR, 1998. Proceedings ISTRC-AB, Malawi 1995 (in press).