Effect of local inoculum on the spread of sweet potato virus disease: limited infection of susceptible cultivars following widespread cultivation of a resistant sweet potato cultivar

A study compared the spread of sweet potato virus disease (SPVD) into crops of two moderately resistant and initially SPVD-free sweet potato cultivars in northern and southern Mpigi, Uganda. Whiteflies, the vector of sweet po tato chlorotic stunt crini virus (SPCSV), a component cause of SPVD, were s imilarly abundant in farmers' sweet potato fields around Namulonge in north ern Mpigi, and Kanoni in southern Mpigi. However, mean incidence of SPVD in farmers' crops neighbouring the trials was higher at Kanoni (13.3%) than a t Namulonge (2.8%). Furthermore, spread of SPVD into initially SPVD-free sw eet potato plots of two only moderately resistant cultivars was greater in plots at Kanoni than in plots at Namulonge. The SPVD-resistant New Kawogo w as the most common cultivar grown in farmers' fields at Namulonge and had f ew diseased plants, whereas susceptible cultivars with relatively high inci dences of disease predominated at Kanoni. Final SPVD incidence in each tria l was positively correlated with a measure combining the proximity and leve l of inoculum in surrounding fields. The study demonstrates the importance of local SPVD inoculum in determining the rate of spread of the disease int o fields and implies that the widespread cultivation of a resistant variety limits infection of susceptible cultivars grown nearby.