Yield loss and spread patterns of sweet potato virus disease in areas around the Lake Victoria Crescent of Uganda

Sweet potato virus disease (SPVD), the main disease constraint to sweet potato production in the area within the Lake Victoria Crescent (LVC), was first reported in Uganda in 1944 and since then, the disease has been reported to cause considerable damage to sweet potato crops. Studies were thus conducted to assess yield reductions and temporal spread patterns of SPVD within the LVC if Uganda. To achieve these objectives, two studies were conducted at Kabanyolo and Mukono for two seasons (2002B1 and 2003A2 seasons), beginning the second season of 2002. The first study compared yields obtained from sweet potato plants infected by cutting and subsequent vector-induced spread. It also compared temporal vector spread effects of SPVD on yield of a susceptible sweet potato cultivar Zapallo. The study revealed that SPVD was severe on sweet potato plants, although at varying levels. SPVD incidences were low during the wet months and high in the dry months. SPVD incidences as high as 60% were recorded at Kabanyolo, whole incidence in Mukono was generally lower than at Kabanyolo. In symptomatic plants, sweet potato chlorotic stunt crinivirus (SPFMV) were the two most frequently detected viruses at both locations and in the two seasons. Mode of SPVD infection had a significant (P<0.05) effect on sweet potato yield. infection by cutting resulted in more reduced yields (4.1t/ha), whereas vector-induced infection resulted in less reduced yields (13.5t/ha), when compared to 22.7t/ha obtained from healthy plants. In most cases, infections by vectors even when it occurred early depressed total storage yield less than planting infected cuttings. Time of SPVD infection influenced storage root yield. Early vector-induced infections were associated with lower yields compared to late vector-induced infections. In the second study conducted during October 2002-September 2003, the development of SPVD was monitored in different months in order to get changes in the infectivity of the vector populations with seasons. SPVD spread in sweet potato plant was recorded for a year on a moderately susceptible cultivar, Tanzania, planted once every month for five months. Throughout the period, whitefly populations were monitored and SPVD infection rates for the different months computed and used to calculate the infectivity indices for each month. SPVD was common on plants in December-January and June to July. The high incidences in these months coincided with high whitefly counts at Kabanyolo (R2=0.68), showing that the amount of disease was positively correlated to the prevalence of whitefly vectors. Contrary to this was at Mukono, where only a weak positive relationship (R2=0.04) was encountered, indicating that SPVD incidence was not associated with the abundance of whitefly vectors. In this study, aphids were rarely found on plants throughout the study period at both locations, despite high incidences of SPVD. The SPVD infection pattern and indices indicated two distinct spread peaks occurring in the dry periods of December -January and June-July. The high values of the infectivity rates accompanied with high whitefly populations provided high infectivity indices recorded in the study period, indicating the different disease pressures encountered in Uganda during the periods when sweet potato are grown. This study, basing on vector population and SPVD incidence has made recommendations on planting time in order to minimize SPVD effects.