Screening of parental cassava genotypes and generated partial inbreds for resistance to cassava brown streak disease in Uganda
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Damage caused by CBSD associated viruses (CBSV and UCBSV) is increasing in Africa and only limited resources are available to control the spread of the disease. Therefore, more robust and durable methods of controlling these viruses are needed. The development of such methods requires the identification of additional sources of useful virus resistance and a better understanding of resistance mechanisms. Here, I report on generation of new sources of resistance and identification and characterization of resistance sources from Uganda and Tanzania. New sources of resistance were generated through inbreeding of selected parental genotypes (S0) with a premise that the generated partial inbreds (S1) would perform better than their respective non-inbred progenitors in terms of resistance to CBSD. According to the study, the generated partial inbreds showed higher disease index than their respective parents though the difference was not significant (5%) except for Namikonga. However, varying number of asymptomatic genotypes for CBSD was recorded from the generated partial inbreds showing that new sources of resistance can be generated through inbreeding. Identification and characterisation of resistance to CBSD from the already available germplasm was done by both laboratory (using real time PCR) and field screening of selected genotypes at National Crops Resources Research Institute (NaCRRI), a “hotspot” for CBSD in Uganda. Ten cassava parental genotypes (three from Uganda and seven from Tanzania) were selected for evaluation. According to the results, none of these screened genotypes exhibited resistance to infection to both UCBSV and CBSV in Uganda. However one genotype (Namikonga, a cross between M. esculenta×M. glaziovii) from Tanzania was found to have comprehensive field resistance to CBSD while NASE 1 from Uganda had tolerance to CBSD. These genotypes were characterized by ranking them based on their disease index and Wilcoxon ranking, a new method developed by this study. The screened genotypes have exhibited various mechanisms of resistance to virus which included resistance to virus accumulation in NASE 14 and reversion or ‘recovery’ from UCBSV infection in Kiroba. In conclusion, new sources of resistance to CBSD of resistance were generated through inbreeding and four genotypes (Namikonga, NASE 1, NASE 14 and Kiroba) among those screened have been identified to have unique attributes that can be exploited for resistance to CBSD.