QTL Associated with Field Resistance to Cassava Brown Streak Disease in the Context of the Genetic Relationship Among Genotypes

In Africa cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) together cause over $1billion losses in production every year in the largely subsistence crop of cassava. These losses are predicted to escalate as CBSD spreads from East towards West Africa, the largest cassava producing region in Africa. Marker-assisted selection (MAS) for resistance to both diseases would enable pre-emptive breeding in West Africa in advance of the CBSD pandemic, and contribute efficiencies in breeding in East Africa. Field resistance to CBSD has been observed in landraces from Tanzania, possibly derived from an early breeding program at Amani (1930-40’s) which included interspecific crosses. QTL studies based on bi-parental crosses involving this germplasm, genotyped using genotyping-by-sequencing and phenotyped over two seasons in two locations in Tanzania have identified different QTL on chromosome 11 in two sources of CBSD resistance, namely ‘Namikonga’ and ‘Kiroba’. Results are supported by the genetic relationships of germplasm based on whole genome shotgun sequencing and results of a GWAS study conducted in Uganda. CBSD resistance does not appear to come from wild species introgression but, at least the resistance from ‘Namikonga’, appears to come from a West African landrace, ‘TMEB117’. Efforts are underway to convert results into practical MAS.