Abstract:
Cassava brown streak disease (CBSD) caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown
streak virus (UCBSV) is a threat to food security in sub-Saharan Africa, where the disease persistently reduces
overall root quality and quantity resulting in up to 100% yield losses. Complexities in CBSD symptom expression
and the damage caused on leaves, stems and roots throughout the 12 months of cassava growth require that
appropriate ways of categorizing genotype response and optimal stages of evaluation be identified. This study
aimed at: 1) determining plot based heritability of CBSD based on symptom expression and 2) categorizing
genotype resistance to CBSD based on symptom expression. Herein, 41 genotypes were evaluated for two years
at Namulonge with an additional evaluation conducted across three locations. Evaluations were done at three,
six, nine and twelve months after planting. Genotype responses to CBSD varied significantly. High broad sense
heritability estimates of up to 0.81 (incidence) and 0.71 (severity) were obtained.
Average disease severity scores had higher broad sense heritability estimates (0.53 and 0.65) than maximum
disease severity scores (0.33 and 0.61) for root and foliar severities respectively. These findings are important in
choosing an appropriate evaluation method for CBSD. Genotypes displayed differing CBSD responses in type,
locality and severity of symptoms. This suggested that genotypes had differences in mechanisms of resistance
that can be exploited in CBSD resistance breeding.
