Abstract:

In Uganda, it is suspected that late blight (Phytophthora infestans) 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, yield losses due to the disease, and little is known about the population structure of P. infestans. This study was therefore, undertaken to: a) establish the variability in occurrence of late blight disease and yield losses due to the diseasre in major potato growing areas of Uganda, and b)characterise P. infestans 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 Kisoro, Kabale, 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 season of 1998. if 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 of 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 was only a weak 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. Eighty one P. infestans isolates recoved from potato samples from Kisoro, Kabale, Mbarara and Mbale were tested for their sensitivity to metalaxyl. Overall 44.4% (36 of 81) of the isolates tested were resistant to metalaxyl, 23.5% intermediate and 31.2% sensitive. In the mating type test, using a known Al (1724) isolate, 50.0 % of the isolates produced oospores in matings and selfings, 22.5% in mating only and 10% in selfing only; 18% did not produce oospores in the two tests. A total of 27 (44.2%) of the 61 P. infestans isolates from potato infected tomato leaves of which, the majority were highly resistant to metalaxyl and produced oospores in either mating or selfing or both. The existence of metalaxyl resistant isolates is an indication of the possible existence of new P. infestans population in Uganda. Most of the metalaxyl resistant isolates werefrom those districts using the fungicide ridomil. The presence of oospores in cultures suggests the potential for hybridisation and selfing of P. infestans in potato growing areas of Uganda, but does not confirm the existence of A2 mating type. The isolates need to be crossed with A2 tester isolate, and isozyme and DNA fingerprinting are needed to confirm the presence of the A2 mating type.