Abstract:
Experiments to determine the effect of the three viruses most common on Kenyan sweet potato
varieties were carried out at the University of Nairobi Kabete farm. The sweet potato varieties were
Bungoma, Kemb 10 and Ex-Shimba hills which were either virus-free or infected with Sweet potato
feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic
stunt virus (SPCSV) or a combination of SPFMV and SPCSV. Canopy photosynthetically active
radiation (PAR) interception and the relative chlorophyll content of the plants were determined. At
harvest, marketable tuber yield, harvest index (HI), leaf area index (LAI) and specific leaf weight
(SLW) were measured.
Infection with single viruses caused no or mild symptoms and had no significant effect on chlorophyll
content of leaves of any variety. Dual infection with SPFMV and SPCSV resulted in severe symptoms
characteristic of sweet potato virus disease complex (SPVD) and significantly reduced chlorophyll
content. Only SPCSV and SPCSV + SPFMV reduced PAR interception, the latter combination
especially. Generally, infection with single viruses increased the SLW but a decrease occurred in
plants dually infected with SPFMV and SPCSV. Whereas SPVD significantly reduced the harvest
index (78%) and tuber yield (98%) in the three varieties, infection with single viruses caused a reduction
only in the varieties Bungoma and Kemb 10. SPVD reduced yield through reducing leaf chlorophyll
content, PAR interception, LAI and HI. The single virus infections possibly reduced assimilate
translocation from leaves to tubers as is inferred from high SLW. Effects of viruses on physiological
processes and yield are discussed.
