Characterization of Phytophthora infestans strains causing late blight provides new disease prevention strategies

Late blight is caused by an increasingly large number of Phytophthora infestans strains that are responsible for acute and chronic disease losses in Canada and the United States on tomato and potato. This historically infamous disease associated with the Irish Potato Famine continues to cause worldwide crop losses each year. A national survey conducted in Canada during 2014 and 2015 collected over 100 P. infestans isolates from the major potato and tomato growing areas. Volumetric spore traps were used to measure pathogen pressures. This data was successfully used to predict the risk of late blight in a given area and to triangulate late blight hot spots that were subsequently eliminated. The P. infestans genotype US-23 dominated late blight pathogen populations in the provinces of Manitoba, New Brunswick, and Ontario and was recovered from both tomatoes and potatoes. Fungicide screening showed that all P. infestans US-23 genotypes were sensitive to the systemic fungicide metalaxyl early in the growing season. Several isolates of the P. infestans genotype US-22 were recovered from tomato samples from southern Ontario. Other P. infestans genotypes isolated included US-8 in British Columbia and US-24 in Quebec but only from potato. In some areas of British Columbia and Ontario, both A1 (US-23) and A2 (US-8 or US-22, respectively) mating types of P. infestans were found in close proximity. This raises concerns because it provides an opportunity for sexual recombination which may lead to the spread of new more damaging genotypes. The oospores produced by P. infestans during sexual reproduction are thick walled pigmented resting spores capable of surviving in the absence of a host for many years. This can shift late blight from its present status in Canada as a sporadic problem to becoming an endemic pest.