Potential of phytosanitation in the management of cassava mosaic disease and sweet potato virus disease in Tanzania

Cassava mosaic disease (CMD) and sweet potato virus disease (SPVD) are among the most important cassava and sweet potato limitations in Tanzania. Field studies were conducted at four locations in Kagera Region in northwestern Tanzania to investigate the potential of phytosanitation in managing CMD and SPVD. The trails were set in areas with contrasting disease-spread conditions using varieties differing in the level of resistance of CMD and SPVD infection. The study had the following specific objectives: (i) to assess the effectiveness of phytosanitation practices for controlling CMD and SPVD under contrasting spread conditions, (ii) to determine the effectiveness of selection and rouging in maintain the health status of plantings when used singly and also in combination. (iii) To characterize the resistance to CMD and SPVD of the main local and improved varieties of cassava and sweet potato respectively in Bukoba and Muleba districts. Results of the study to assess the effectiveness of phytosanitation practices revealed significant differences (P<0.001) in CMD incidence at the end of the experiment among location, varieties and seasons. At harvest, no significant differences were observed in disease incidence between disease management practices despite the fact that rouging of infected plants reduced disease incidence significantly at all location (P<0.05) at rouging time, i.e four weeks after planting, overall higher disease incidences in the susceptible varieties were noted at the location which were determined to have high disease pressure conditions for both cassava and sweet potato trials. Specifically, Kyaka had higher SPVD incidences than Maruku and Ngenge in the sweet potato phytosanitation trial, while Nsunga and Ngenge had higher CMD incidences as compared to Maruku in the cassava phytosanitation trial. The different management did not significantly (P>0.05) affect tuber yield in either cassava or sweet potato phytosanitation trials. In the study to characterize varieties for esistance to CMD ana SPVD, varieties were found to react differently to both CMD and SPVD infection. For the cassava trial, local landraces (i.e. Rushula, Kaitampununyeupe and Mukarukwatage) were more affected byCMD than the introduced varieties [i.e. MM 96/8233, SS4, Migyera AND tms 4(2) 1425]. Significantly differences were noted among varieties and growing seasons, which implied that the effects of growing season and variety influenced virus infection levels. In cassava trial, significant 9P, 0.001) differences were observed in plnat height (m), storage root number per plant, and CMD incidence and severity at 12 MAP, but not in storage root weight per plant. In sweet potato evaluation trial, two varieties (Polista and Hidaya) exhibited some resistance to SPVD, but the officially released sweet potato varieties (Simama, Mavuno and Juhudi) were significantly more affected by SPVD an all growing seasons. Then results from this study suggest that phytosanitary practices may not work when used alone. Level of varietal resistance, disease inoculum pressure in the locality and time of planting need to be taken into account when recommending the use of phytosanitary practices to control CMD and SPVD in farmers’ fields.