Occurrence of sweet potato to virus disease (spvd) and other viruses on sweet potato and the role of resistance and phytosanitation to control spvd in Uganda

This study was conducted to establish the prevalence and distribution of sweetpotato virus disease and other viruses infecting sweetpotato in Uganda, assess the level of resistance in the most commonly grown sweetpotato landraces in areas of low and high SPVD incidences, and the effect of roguing on sweepotato tuber in Uganda. The results of the diagnostic survey (IDS) conducted between November 2000 and January 2001 showed that sweetpotato virus disease occurs in all districts but with varying incidences. In general, there were significant variations in incidences of SPVD amongst fields within the sub-counties and sometime within districts. Out of the 338 fields visited, 66% had plants with SPVD symptoms. The highest and lowest SPVD incidences were reduced in the western and northern regions, respectively. Serodiagnosis detected five viruses namely; SPEMV, SPCSV, SPMMV, SPLV and SPFCV from the samples collected during the survey. Overall, these viruses occurred as multiple infections rather than a single infection. In the study to assess the level of resistance in sweetpotato landraces, ten sweetpotato landraces collected form eight districts of Uganda with varying incidences of SPVD were evaluated for three seasons (2001A-2000A) at Namulonge in central Uganda. There were significant differences (P<0.001) in reaction to SPVD among the sweetpotato landraces for all the seasons. In general, sweetpotato landraces form low SPVD incidence areas recorded the highest incidences of SPVD during the three seasons of study compared to those from the high SPVD infection areas. This result therefore indicate that sweetpotato landraces form low SPVD incidence areas should be evaluated in SPVD hotspots areas to confirm their resistance. Overall more disease was observed during the first seasons of 2001 and 2002 compared to the second season of 2001 suggesting that SPVD epidemics is strongly influenced by the environmental conditions. During the three seasons, the highest SPVD incidences of 91.5 and 86.8% were recorded in Araka, and 46% on Ombivu. In addition, there were significant differences (P<0.05) in yield among the sweetpotato landraces, with landraces from high SPVD infection areas giving higher yields than those from the low SPVD infection areas. In the study to examine the influence of rouging on sweetpotato tuber yield,infection levels significantly (P<0.05) influenced fresh vine weight, marketable and total tuber yields. At Serere, the highest and lowest tuber yields were obtained from 50 and 100% infection levels, respectively. However, at Namolonge, the highest and lowest tuber yield during the second season of 2001 was obtained from 25 and 100% infection levels. Respectively although this did not differ significantly from 50 and 75% infection levels. In general, considerable compensatory effects of rouging on sweetpotato tuber yield were observed amongst treatments although this varied with the proportions of healthy plants remaining. This, when infected plants are rogued off, the remaining plants will have the capacity to compensate for the lost plants in terms of total tuber yield and other parameters. However, in areas of high SPVD spread and high vector activity rouging may not provide effective control against SPVD compared to the low SPVD pressure zones.