Characterization of the potato late blight pathogen (Phytophthora infestans) in Canada and its implications on disease management and fungicide resistance

Abstract

Late blight, caused by Phytophthora infestans, poses a major threat to potato and tomato crops globally. In Canada, increased incidence and severity of late blight have coincided with shifts in the genetic makeup of P. infestans. The aim of this research was to track the distribution of P. infestans isolates in Canada and characterize their phenotypic and genotypic traits including underlying fungicide resistance mechanisms. Overall, the study identified four major existing genotypes (US11, US17, US8, and US23) along with 26 novel genotypes across Canada. The US11 genotype dominated in Pacific western Canada, while US23 was prevalent in eastern Canada and US17 was identified for the first time in Canada. A higher incidence of late blight and greater genetic diversity was observed in western Canada compared to eastern Canada evidenced by the number of multilocus genotypes and genetic diversity indices. Also, increased insensitivity of the US8 genotype to fluazinam, chlorothalonil, fenamidone, and metalaxyl was observed in vitro (agar plate and spore germination assays) with IC50 values between 7 and 71 µg/ml whereas US23 remained generally sensitive. In the whole plant assay, the fungicide treatments effectively reduced late blight severity (~ 90 %) caused by US23 both preventatively and curatively. The fungicides also inhibited oospore production, except for Ridomil. Untargeted metabolomic analysis using GC×GC-TOFMS revealed distinct changes in fatty acids, sugars, amino acids, and organic compound concentrations, particularly in metalaxyl-amended samples. The potential fungicide resistance pathways include membrane fluidity, biosynthesis of fatty acids, glycerophospholipid metabolism and cell wall thickening. Continuous monitoring of P. infestans populations, particularly in Pacific western Canada, is essential due to the emergence of novel genotypes and high levels of fungicide resistance. The findings in this study underscore the importance of continuous assessment of P. infestans sensitivity to commonly used fungicides in Canada. Resistance to these fungicides could limit management options and complicate disease control efforts.