Genetic Composition of Phytophthora infestans in Canada Reveals Migration and Increased Diversity

Abstract

A dramatic increase in the incidence of late blight and changes within populations of Phytophthora infestans have been observed in various regions of Canada. In this study, the occurrence of several new genotypes of the pathogen was documented with associated phenotypes that dominated pathogen populations. Genotype US-23, previously detected only among isolates from the United States, dominated in the western Canadian provinces of British Columbia, Alberta (AB), Saskatchewan, and Manitoba (MB). Although isolates of US-23 infect both potato and tomato, these isolates were the only genotype recovered from commercial garden centers in Canada. Isolates of genotype US-8, previously dominant throughout Canada, represented the only genotype detected from the eastern Canadian provinces of New Brunswick and Prince Edward Island. Isolates of other genotypes detected in Canada included US-11 in AB, US-24 in MB, and US-22 in Ontario (ON). An additional genotype was detected in ON which appears to be a derivative of US-22 that may have arisen through sexual reproduction. However, evidence of clonal reproduction dominated among the isolates collected, and opportunities for sexual reproduction were probably limited because of a surprising geographic separation of the A1 and A2 mating types in Canada. Sensitivity of the US-22, US-23, and US-24 isolates to the fungicide metalaxyl, movement of potato seed and transplants, and weather conditions may have contributed to reduced opportunities for contact between the mating types in fields in Canada. All P. infestans isolates were readily distinguished from other related oomycetes with RG57 restriction fragment length polymorphism analysis. Long-distance movement in seed tubers and garden center transplants may have contributed to the rapid spread of the P. infestans genotypes across Canada. Tracking pathogen movement and population composition should improve the ability to predict the genotypes expected each year in different regions of Canada.

Evolution and Management of the Irish Potato Famine Pathogen Phytophthora Infestans in Canada and the United States

“…3). Moreover, we have recently observed multiple recombination events in several P. infestans isolates from Ontario, in addition to the A1 and A2 mating types, found in relatively close proximity on tomatoes (Kalischuk et al 2012;Peters et al 2014). Perhaps tomato is a preferred host for increased P. infestans recombination and the generation of new genotypes with challenging characteristics.…”

“…Genetic analysis of P. infestans populations has facilitated a proactive approach in the application of sustainable disease management practices that involve the elimination of specific pathogen genotypes and the selection of effective fungicides. Phenotypic markers available for characterizing P. infestans isolates include mating type (A1 or A2) (Gallegly and Galindo 1958), virulence (Malcolmson and Black 1966;Malcolmson 1969;Cooke et al 2003), host preference (Kalischuk et al 2012;Danies et al 2013), and resistance or sensitivity to the fungicide metalaxyl (Dowley and O'Sullivan 1981;Fry et al 1993).…”

“…For example, US-1.2, US-1.3 US-1.5, and US-1.6 variants within the US-1 clonal lineage of P. infestans might have originated from mutations at DNA fingerprint loci, while US-1.1 and US-1.4 variants might have originated from mitotic recombination at DNA fingerprint loci that were heterozygous (Goodwin 1997;Goodwin et al 1994a). The variations within a clonal lineage resulted in characteristic changes such as aggressiveness, growth rate, and colony morphology which allowed for selection by evolutionary processes, such that the selected lines proliferated while the other variants eventually disappeared (Goodwin 1997;Kalischuk et al 2012). The US-6 genotype was introduced into the United States and Canada in the late 1980s from northwestern Mexico near Los Mochis, Sinaloa (Fry and Goodwin 1997b;Smart and Fry 2001).…”

“…Between 1995 and 1998, the P. infestans US-18 and US-19 genotypes (both sensitive to metalaxyl) were affecting tomatoes in North Carolina, while the US-17 genotype was the predominant genotype affecting tomatoes elsewhere in the United States (Fraser et al 1999;Wangsomnoondee et al 2002). Recently, the dominant P. infestans genotypes on tomato have been US-22 and US-23 which were initially sensitive to metalaxyl but developed insensitivity within a few years (Kalischuk et al 2012;Peters et al 2014).…”

“…The pathogen is capable of penetrating a healthy tuber periderm which reduces yield and quality and diminishes storability by facilitating secondary infections . The pathogen reproduces predominantly by asexual means in the United States and Canada via sporangia formation on infected host tissues which are readily dispersed by wind and rain Hu et al 2012;Kalischuk et al 2012). These sporangia then directly initiate infection or form motile zoospores.…”