Prevalence and co-infection of cassava with cassava mosaic geminiviruses and cassava brown streak virus in popular cultivars in Western Kenya
Cassava mosaic gemtntviruses (CMGs) and Cassava brown streak virus (CBSV) cause diseases of major economic significance to cassava in Kenya. The Western region of Kenya ,contributes about 60% of cassava production in the country. Currently CBSD distribution is ,being newly reported in the Western region where over 90% CMD incidences have been observed. Transmission studies have confirmed that B. tabaci transmits both CMGs and 'CBSV. The co-occurrence of the vector, CMGs and the newly reported CBSV undoubtedly raises the alarming possibility of a 'dual pandemic'. This study was designed to investigate the dual virus spread, the possible interactions of CMGs and CBSV on the host plant and their effect on the development of symptoms on the popular cassava varieties. To evaluate the status of the two viruses on the local popular varieties in terms of incidence, prevalence and severity, a survey was conducted in Kakamega, Teso, Bungoma, Siaya and Busia districts in Western Kenya and 273 samples collected from 67 farmer's fields. The CMGs and CBSV were confirmed by polymerase chain reaction (PCR) and reverse transcriptase polymerase chain reaction (RT- PCR) respectively. The CMD incidence among the varieties was highest in Embwanatereka 89%, Serere 87%, Adhiambolera 86%, and lowest in Kibandameno 83% while CBSD incidence was highest in Serere 73%, Adhiambolera 69%, and Kibandameno 56% and lowest in Embwanatereka 51 %. To assess the possible interactions of the two viruses, 10 Nicotiana benthamiana plants were sap inoculated with CMGs and CBSV singly and in combination in two trials. A highly significant difference (P<0.05) was observed in the interaction of the virus and time. At 30 days post inoculation, dual infection of CMD+CBSD showed the highest severity score (4.7) followed by CBSV (4.1) and CMD (3), suggesting synergistic interaction of the two viruses when occurring together. To determine effect of dual infection of CBSV and CMGs species in 4 local popular cultivars, 10 replicates from each cassava landrace were graft-inoculated with diseased scions and foliar symptoms examined using a scale of 1-5. No significant difference (P>0.05) on mean severity was observed when each virus occurred alone in the cassava landraces. However, in combination of CMD and CBSD, significant difference (P<0.05) were observed on mean severity among the cassava landraces. The study provides the first report of the presence and foliar effect of dual infection ofCMD and CBSD in popular cassava landraces and possible interaction of CMD and CBSD. The study further gives evidence of possible synergistic interaction between CMD and CBSD. The high level of the virus disease incidences observed indicates an urgent need to deploy control strategies in Western Kenya.
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