Variability in late blight severity and pathogen population in Uganda

In Uganda, it is suspected that late blight (Phytophora infestants) is largely responsible for the low tuber yields of potato. However, there is no quantitative information on the variability in occurrence of late blight disease in the country, yields losses due to the disease, and little is known about the population structure of P. infestants. This study was therefore, undertaken to: a) establish the variability in occurrence of the late blight disease and yield losses due to the disease in major potato growing areas of Uganda, and b) characterise P. infestants populations in Uganda with respect to occurrence of A2 mating type, pathogen resistance to metalaxyl, and cross infection between potato and tomato. The first part of the study involved field surveys in the districts of Kabale, Kisoro, Mbarara and Mbale. Potato fields were assessed for late blight incidence and severity. Information on type of fungicide used, number of sprays, and names of cultivars were also noted. Yield loss assessments were done in farmers fields in the districts of Kabale and Mbarara during the second session of 1998. Late blight incidence and severity varied significantly (P≤0.05) from district to district and season to season during the four surveys. Overall, Kabale district consistently recorded the highest late blight severities, with mean severity of 27.6%, followed by Mbale (21.8%), and Mbarara the least severity (10.8%). There was more disease during December 1997, with mean severity (40.2%), while the least severity was in May 1998. The highest severity was recorded on variety Victoria (62.1%)followed by Wafrika (61.8%). Sangema (59.5%), Maboni (48.2%). Nevertheless, there only a week negative correlation between late blight severity and yield of potatoes, likely because the yield loss assessment was done during a season of low late blight occurrence.