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.
Highlights
Cassava (Manihot esculenta Crantz.) is affected by cassava brown streak disease, one of the seven most serious threats to food security in the world (Pennisi, 2010)
Further spread and occurrence of CBSD has been confirmed in Burundi (Bigirimana et al, 2011) and eastern Democratic Republic of Congo (DRC) (Mulimbi et al, 2012), with the most recent outbreaks reported as far as Gabon and Angola (FAO, 2013)
The differences observed in CBSD symptom expression in different plant parts with time creates a need to develop a universal approach of estimating levels of resistance based on symptom expression. For this reason the current study focused on categorizing genotype resistance to CBSD based on symptom expression
Summary
Cassava (Manihot esculenta Crantz.) is affected by cassava brown streak disease, one of the seven most serious threats to food security in the world (Pennisi, 2010). The most recent study has shown that, in addition to the two species (CBSV and UCBSV), three clades within UCBSV exist, indicating the possibility of four distinct species of CBSD causative viruses (Ndunguru et al, 2015). These viruses are transmitted by the whitefly Bemisia tabaci as a vector (Maruthi et al, 2005; Mware et al, 2009). Spreader rows of TME 204, a highly susceptible variety (Kaweesi et al, 2016), were planted after every five rows to augment CBSD disease pressure. CBSD susceptible (TME 204) and tolerant (NASE 14) genotypes were included as checks for comparison purposes
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