Begomoviruses infecting cassava and their interaction with brown streak viruses in Kenya

Abstract

Cassava mosaic begomoviruses are known to occur together within the same host and have a high rate of recombination. CMBs have also been reported to co-infect cassava with CBSVs, resulting in enhanced symptom expression. Interaction of virus species within the same group may result in recombination or synergism while interaction between the different groups may result in synergism. This dissertation determined the diversity and level of co-infection of CMBs in Kenya and also examined the nature of interaction of East African cassava mosaic virusUganda (a variant of EACMV: a CMB), with CBSVs. For the first objective, a survey was conducted in the four major cassava growing regions of Kenya namely Eastern, Coastal, Nyanza and Western. The study reported for the first time the occurrence of East African cassava mosaic Cameroon virus (EACMCV) in all cassava growing regions and a high (82.96%) level of coinfection of all CMBs in Kenya. East African cassava mosaic Zanzibar virus (EACMZV) was the most prevalent CMB in all cassava growing regions and had spread from the coastal region where it previously occurred. For the second objective, two CMD resistant varieties and one susceptible variety were used to study the interaction of East African cassava mosaic virusUganda (EACMV-Ug) with the CBSVs: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). This study was conducted in a greenhouse at Jomo Kenyatta University of Agriculture and Technology in Kenya. Quantitative RT-PCR and PCR analysis were performed to compare viral titre in single and co-infection. In the susceptible variety, Mucericeri, EACMV-Ug was higher when in co-infection with CBSV but lower when in co-infection with UCBSV, compared to single infection. In CMD resistant variety, MM 96/5280, the titre of EACMV-Ug at 3 mpi was raised significantly by both CBSV and UCBSV. For CBSVs, the titre of UCBSV was lower at 3 mpi but higher at 6mpi when in co-infection in the susceptible variety Mucericeri. EACMV-Ug increased the viral titre of CBSV in MH 99/7296. Detection of a previously unreported CMB and the synergism of CMB in CMD resistant varieties call for stringent diagnosis of cassava viruses and development of new sources of resistance to CMD. The findings in this dissertation contribute to the knowledge gap on the complex nature of virus interaction and will contribute to the long term goal of managing and screening cassava viruses.