Effects of viruses on growth and yield of local sweet potato varieties in Kenya
Experiments to determine the effect of viruses on the vine and tuber yield of three popular Kenya sweet potato varieties were undertaken at the University of Nairobi Kabete farm, The sweet potato varieties were Kemb10, Bungoma and Ex-shimba hills which were either virus free or infected with sweet potato feathery mottle potyvirus (SPFMV), sweet potato mild mottle ipomovirus (SPMMV), sweet potato chlorotic stunt crinivirus (SPCSV) or SPFMV + SPCSV. The treatment combination was a split plot factorial structure in a randomized complete block design with three replicates. Disease severity and photosynthetically active radiation (PAR) interception by the plant canopy were determined. At harvest, leaf area index (LAI), specific leaf weight (SL W), fresh vine weight, total tuber yield, marketable and unmarketable tuber yield and harvest index (HI) were determined. Dual infection resulted in severe symptoms characteristic of sweet potato virus disease complex (SPVD). SPFMV and SPMMV had no significant effect on PAR intercepted by the plant canopy. Infection with SPCSV and SPFMV + SPCSV had a reduced PAR interception by 18% and 75% respectively. In addition, SPCSV and dual infection reduced fresh vine yield by 40% and 75% respectively. Infection with single viruses caused an increase in SLW ranging from 5 to 40% while plants dually infected with SPFMV + SPCSV reduced SLW by 15%. Only dual infection with SPFMV + SPCSV caused 66% reduction in LAI. Plants dually infected with SPFMV + SPCSV caused a marketable tuber yield loss ranging from 82 - 98% in all varieties. Infection with SPFMV, SPCSV and SPMMV on varieties Bungoma and Kemb 10 reduced yields through hindered translocation of photosynthate as inferred by increased IV SLW and reduced HI. Variety Ex-shimba hills was found to be tolerant to single virus infections but Bungoma and Kemb 10 were susceptible. All the three varieties were susceptible to' 'infection with SPFMV + SPCSV causing reduced vine' and tuber yield through reduced LAI, reduced photosynthetic organs due to stunting, chlorosis and consequently PAR interception by plant canopy. Dual infection also caused a reduction in tuber yield through harvest index.