Evaluation of a cassava germplasm collection for reaction to three major diseases and the effect on yield

Thirty-five cassava (Manihot esculenta Crantz) genotypes from the Internati onal Institute of Tropical Agriculture (IITA), germplasm collection were pl anted in May 1992 under natural disease infection conditions, at the IITA f ield plots at Ibadan, Nigeria. The genotypes were evaluated at monthly inte rvals for six months, for incidence and severity of cassava anthracnose dis ease (CAD), cassava bacterial blight (CBB) and African cassava mosaic virus (ACMV). The effect of the pathogens on yield was also evaluated twelve mon ths after planting (MAP). CAD, CBB and ACMV incidence and severity were sig nificantly different (P <0.05) among the cassava genotypes at 3 and 6 month s after planting. ACMV incidence and severity reached the maximum levels at 3 MAP while CAD and CBB had a low incidence and severity at that period bu t exhibited a high disease symptom expression at 6 MAP. Yield parameters (p lant stand, tuber number, tuber weight and tuber dry matter) differed signi ficantly (P <0.05) between the cassava genotypes. ACMV and CBB severity was negatively correlated with tuber number (r = -0.54 and r = -0.62 respectiv ely). CAD severity was negatively correlated with tuber number (r = -0.46) and tuber weight (r = -0.63). Percentage dry matter was significantly corre lated with ACMV severity (r = 0.71), CBB severity (r = 0.63) and CAD severi ty (r = 0.42). There was also a significant correlation between CBB and ACM D severity (r = 0.46) and CBB and CAD severity (r = 0.62). Tuber number and yield were significantly correlated (r = 0.60), while% dry matter content and root rot showed a significant negative correlation (-0.42).