Abstract:
Cassava mosaic geminiviruses (CMGs) and cassava brown streak viruses (CBSVs) respectively cause cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) which significantly inhibit cassava production in Kenya. This study aimed at determining the prevalence and incidence of CMD and CBSD in lower eastern Kenya and resistance response of new improved cassava genotypes. To determine the prevalence and incidence of CMD and CBSD in lower eastern Kenya, survey was conducted in Machakos, Makueni and Kitui and thirty farmer’s field were sampled. Sampling procedure involved stopping at regular intervals of about 15 to 20km between farmer’s fields along transect in each sampling location and sampled farms purposively selected. To evaluate the resistance response of fifteen improved genotypes and two local susceptible controls, field experiment was laid out in randomized complete block design with four replicates. Three replicates from each improved and susceptible controls were further chip bud grafted with CMBs and CBSVs infected scions under greenhouse conditions in a complete randomized design. Results revealed 51% CMD and 17.67% CBSD incidences in lower Eastern Kenya. Both diseases exhibited wider incidence distributions with 59%, 63% and 31% CMD recorded in Makueni, Kitui and Machakos respectively, while 31%, 15% and 7% CBSD was observed in the same order. Bemisia tabaci, exhibited wider distributions with an average of three flies per plant in Makueni, two in Kitui and one in Machakos. All fifteen improved genotypes did not show any viral disease symptoms while 93.7% and 100% CMD and CBSD incidences respectively, were analyzed in susceptible 990072, while 990067, recorded 100% CMD and 81.3% CBSD incidences under field conditions. Variations in CBSD and CMD symptom were observed under greenhouse conditions, For example only four of the improved genotypes (Kiboko281, Thika280, Kiboko277 and Kiboko276) and two controls (990072 and 990067) expressed between 20 – 90% CBSD incidences from 6 – 8 weeks after grafting. In contrast, ten genotypes and two controls (990072 and 990067) exhibited 10 – 60% CMD incidences between 2 – 8 weeks after grafting. The study gives evidence of the presence of both viral diseases in lower eastern Kenya and recommends that the five (Kiboko300, Kiboko297, Thika272, Thika279 and Kiboko275) improved genotypes found to be CMD and CBSD free be adopted by farmers in lower eastern Kenya.
