Phenotypes ofPhytophthora infestans in Nepal: mating types and metalaxyl sensitivity

Summary Potato late blight has appeared in epidemic proportions in Nepal since the mid 1990s and fungicides have been reported to be decreasingly effective in managing the disease.Phytophthora infestans isolates were collected from potato crops during 1999–2000 and analysis of 371 isolates for mating types and 270 isolates for metalaxyl sensitivity revealed the presence of both new and old populations. This is the first report on the presence of metalaxyl-resistant isolates in Nepal. The frequencies of A1 and A2 were 83 and 17%, respectively. Metalaxyl-resistant, intermediate and sensitive isolates were recorded as 10%, 12% and 78% respectively. Metalaxyl resistance was distributed in both mating types. Sites with a history of metalaxyl use had a significantly higher number of resistant and intermediate isolates ({ie337-1}, P<0.01) than sites where metalaxyl had not been used. This study confirms the changes in population structure ofP. infestans in Nepal.