Effect of planting season on cassava anthracnose disease development

Twelve cassava (Manihot esculenta Crantz) genotypes were planted under natu ral field conditions for two different cropping seasons (wet and dry), in 1 992 and 1993 at the research farm of the International Institute of Tropica l Agriculture (IITA), Ibadan, Nigeria. The trial studied the effect of plan ting season on cassava anthracnose disease (CAD) symptoms expression (disea se severity index, canker size and first canker distance from the ground). Another trial was carried out to investigate genotype susceptibility over t hree consecutive planting seasons using CAD incidence and severity. Disease severity index (DSI) differed significantly (p less than or equal to 0.05) among cassava genotypes in both wet and dry season planting. CAD incidence and severity also differed significantly (p less than or equal to 0.05) am ong genotypes, with the wet season recording higher disease incidence and c anker size. CAD incidence and severity showed significant correlation in th e dry season (r = 0.91), but were negatively correlated (r = -0.87) in the wet season. Necrotic lesion size and disease severity in the dry season wer e not significantly correlated (r = 0.23). Differences in genotypic reactio n to CAD incidence and severity were found in 1992, 1993 and 1994 plantings . Genotypes 30211 and 91/00684 showed more stable resistance over the three years of planting when compared with genotypes 90/00333, 91/00396 and 89/0 0011 which showed fluctuations in CAD disease symptom expression. Yield ass essment of cassava genotypes during the cropping seasons, over the 2-year p eriod, showed great variation in plant stand, tuber number and weight and p ercent dry matter content. (C) 1999 Elsevier Science Ltd. All rights reserv ed.