QTL Mapping for Resistance to Cassava Viruses in Tanzania

Cassava viruses cause extensive damage to cassava in Africa and are a major threat to food security. Here we report progress on a project to map quantitative trait loci (QTL) for resistance or tolerance to cassava brown streak disease (CBSD) in five varieties and to cassava mosaic disease (CMD) in one variety in Tanzania. Five biparental F1 mapping populations were developed, namely Namikonga x Albert, Nachinyaya x AR37-80, Kiroba x AR37-80, AR40-6 x Albert and NDL06-132 x AR37-80. All populations were genotyped using genotyping-by-sequencing, and framework genetic linkage maps developed. For most populations, phenotyping has been conducted in two locations over a single year, and a second year of phenotyping data is currently being collected in three locations in Tanzania. A preliminary analysis of data from Namikonga x Albert reveals one major QTL explaining 34.9% of variation for CMD resistance in Albert. This QTL was consistent across two phenotyping sites and two years. CBSD foliar and root symptoms revealed different QTL. QTL for CBSD foliar data based on a severity score of 1 (no disease symptoms) to 5 (severe leaf symptoms with die back) explained a maximum of 10.7% of variation, while that for CBSD root necrosis, also scored on a 1-5 scale, accounted for 16.2% of observed variation. Further analysis is underway using a quantitative score for root necrosis, based on digital image analysis. Supporting evidence from other studies implies that markers associated with QTL identified here can be applied in marker-assisted breeding.