Rapid Screening Method of Cassava Cultivars for Resistance to Colletotrichum gloeosporioides f.sp. manihotis

An in vitro method for assessing cassava anthracnose disease (CAD) resistance was developed as a preliminary screen to a CAD-resistant breeding programme. Potato dextrose agar (PDA) media was amended by extracts from the stem cortex of 10 cassava cultivars (30001; 30572, 30211, 88/02549, 88/00695, 88/01336, 91/00344, 91/00313, 91/00684 and 91/00475), and assayed for ecacy of inhibition of the growth of Colletotrichum gloeosporioides f. sp. manihotis isolates (05FCN, 10FCN, 12FCN, and 18FCN). Morphological and physiological data indicated that there was a signi®cant difference (P £ 0.05), in mycelial growth, spore germi- nation and sporulation among the four isolates on PDA amended with cassava stem extracts. Extracts from cassava cultivars 30211, 91/00684 and 91/00313 showed higher inhibition of germ tube development, mycelial growth and sporulation of the fungal isolates, whereas cultivars 88/02549 and 88/01336 showed the least inhibition. The 10 cultivars were further tested in both greenhouse and ®eld conditions, under disease pressure for two planting seasons, to corroborate resistance to the fungus as observed in vitro. Greenhouse and ®eld trials with the 10 cassava cultivars showed a signi®cant difference (P £ 0.05) in CAD resistance. Cultivars 88/02549 and 88/01336 were highly CAD-susceptible, as shown in the in vitro assays and con®rmed in the greenhouse and ®eld tests. The other eight cultivars were either resistant (30211, 91/00684), or moderately resist- ant (30572, 88/00695, 91/00475, 91/00344, 30001 and 91/00313) to CAD. The study shows that an in vitro screening assay of cassava for resistance to CAD could serve as a convenient preliminary screening technique to discriminate CAD-resistant from CAD-susceptible cas- sava cultivars. The in vitro screening method consider- ably reduces time and labour in comparison with the current screening techniques of cassava, which involve ®eld planting, inoculation and evaluation.