Late blight is caused by an increasingly large number of Phytophthora infestans genotypes that are responsible for devastating disease losses in solanaceous crops. A national survey conducted in Canada during 2009 through 2016 produced over 500 P. infestans isolates from the major potato and tomato growing areas. The P. infestans genotype US-23 A1 mating type was first detected in Canada during 2009 and now dominates the late blight pathogen populations on both tomatoes and potatoes. Characteristics of this genotype include drought tolerance, temperature tolerance and proliferation on tomato and potato that contribute to its frequent occurrence in gardens and increasing prevalence in commercial crops. Fungicide screening showed that all P. infestans US-23 genotypes were sensitive to the systemic fungicide metalaxyl early in the growing season but resistance developed rapidly.
Many isolates of the P. infestans genotypes US-22 A2 mating type were recovered but only from tomato samples while US-8 A2 mating type, US-11 A1 mating type, and US-24 A1 mating type were only isolated from potato. In some regions of British Columbia and Ontario, both A1 and A2 mating types of P. infestans were found in close proximity. This raises concerns as it provides an opportunity for the production of thick walled pigmented resting spores that are capable of surviving in the absence of a host for many years and sexual recombination. Evidence of recombination was detected in some pathogen populations from those areas with both P. infestans mating types. Volumetric spore traps were used to measure pathogen pressures to determine the timing and reduce the need for preventative fungicide applications. This data was successfully used to predict the risk of late blight in a given area and to triangulate disease hot spots that were subsequently eliminated.