Abstract

Field based trials were setup to evaluate response and inherent resistance mechanisms tocassava mosaic disease (CMD) of four improved varieties; Nase 9, Nase 11, 00036 and 00057 together with Nase 4 and Bao as resistant and susceptible standards, respectively. These were grown in a CMD epidemic hotspot at Namulonge in Central Uganda. There were differences in sensitivity to CMD and whitefly populations among tested varieties. The improved varieties were less affected by CMD than the susceptible standard Bao. Three resistance mechanisms were exhibited by the varieties tested, namely; low infectibility (00036), recovery (Nase 9 and Nase 11) and reversion. Two of the varieties, that is, Nase 4 and 00057 showed all three resistance mechanisms. High whitefly populations characterized Bao, Nase 4 and 00057. Most of the varieties had a higher infestation of whiteflies on healthy than diseased plants. Results also showed that growth and yield parameters depended on the variety, growth stage at infection and health status of the cuttings used with improved varieties recording lower yield losses than the susceptible Bao. Also, plants infected earlier in the growth period suffered higher yield losses. Furthermore, plots planted from diseased cuttings recorded higher yield losses compared to those planted from healthy cuttings.

Key words: Cassava mosaic disease, improved cassava varieties, resistance mechanisms.