Characterization of Phytophthora infestans population diversity in Canada reveals increased migration and genotype recombination

Late blight, caused by Phytophthora infestans, is a devastating disease of potatoes and tomatoes that causes significant crop losses worldwide. Late blight diversity has been increasing since 2009 in many areas of Canada on both potatoes and tomatoes. Although the US-8 genotype of P. infestans has dominated pathogen populations in much of Canada for the past 15 years, novel genotypes have emerged in recent years. A comprehensive survey of late blight samples collected across Canada in 2011 indicated that although the US-8 genotype of P. infestans still dominated pathogen populations in Prince Edward Island, new US-23 and US-24 genotypes had become established in other parts of Canada. The US-11 and US-22 genotypes dominated populations of the pathogen from specific areas of Alberta and Ontario, respectively. Clonal propagation and migration contributed to long-distance transport of P. infestans genotypes and in some situations, both A1 and A2 mating types of the pathogen were found in the same production region. This finding is of concern because it increases the potential for sexual recombination within pathogen populations, leading to the production of oospores that can survive in the absence of host tissue and create new strains of P. infestans. Evidence for recombination in the P. infestans population was detected in Ontario on tomato. Segregation of Gpi allozymes and mating type appeared to occur independently of the RG57 loci, producing several new P. infestans genotypes. Prediction and management of late blight will need to be evaluated and modified to accommodate these new potentially aggressive P. infestans genotypes. © 2014 The contribution of R. D. Peters.