Characterization of Phytophthora infestans Populations in Western Washington

Abstract

The first detection in the United States of isolates of Phytophthora infestans having metalaxyl insensitivity and complex pathotypes occurred in western Washington during the early 1990s. To determine the genetic structure of the current population in western Washington, a total of 115 isolates of P. infestans were obtained during 1996 from infected tubers or foliage of potato, tomato, nightshade, and bittersweet throughout the region. An additional 45 isolates were collected from a single field. Based on mating type, metalaxyl-insensitivity, and molecular markers (allozymes of glucose-6-phosphate isomerase, peptidase, and RG57 DNA fingerprint), all of the isolates were A1 mating type and had the US-11 multilocus genotype. Analyses of an additional 120 isolates collected during 1997 from potato, tomato, and nightshade were performed. As in 1996, US-11 was the predominant genotype detected on all three hosts. However, three additional A2 mating type genotypes were also detected: US-7, US-8, and US-14. These three genotypes represent the first A2 mating type isolates detected in western Washington. Most of a subset of 60 isolates infected 4 to 7 of the 10 potato differentials tested. This included 90% of the isolates collected in 1996 (all US-11), plus 72% of the US-11 and 100% of the US-8 and US-14 isolates collected during 1997. Virulence phenotypes in this region are complex even without the selection pressure of R-genes in the local commercial cultivars. The apparent increase in genetic variation observed in populations of P. infestans in western Washington from 1996 to 1997 most likely occurred by migration rather than by sexual recombination.

Virulence and aggressiveness of Phytophthora infestans isolates collected in Poland from potato and tomato plants identified no strong specificity

“…A key aspect of our experiment was that the collected isolates (76 P and 100 T) were maintained on their respective host plants to protect their potential adaptive characteristics. In most other studies, P. infestans isolates have been maintained on agar medium, although some have conducted multiplication of isolates on potato tissue Legard et al 1995;Forbes et al 1997;Lebreton and Andrivon 1998;Dorrance et al 1999;Lebreton et al 1999;Vega-Sánchez et al 2000). The loss of pathogenicity of isolates during long-term culturing on agar has been noted by some authors (Wilson and Gallegly 1955;Jinks and Grindle 1963).…”

“…Quantitative assessments of the fitness parameters of P. infestans have been conducted mostly with detached-leaf or leaflet assays (Goodwin et al 1995;Legard et al 1995;Świeżyński et al 2000). Although P. infestans isolates are obtained from both potato and tomato, aggressiveness and virulence tests of the isolates have been evaluated primarily on potato differentials (Lebreton and Andrivon 1998;Dorrance et al 1999;Vega-Sánchez et al 2000;Derie and Inglis 2001;Klarfeld et al 2009;Blandón-Diaz et al 2012;Harbaoui et al 2013). Aggressiveness and virulence of isolates from potato and tomato have also been compared on leaflets of the two host plants (Wilson and Gallegly 1955;Oyarzun et al 1998;Lebreton et al 1999;Jaime-Garcia et al 2000;Cohen 2002; Knapova and Gisi 2002;Blandón-Diaz 2011).…”

“…infestans isolates are often stored on artificial media and/or multiplied on potato tuber slices or leaves before being subjected to host adaptation experiments, even if the original host species of the isolate was tomato (Legard et al 1995;Lebreton and Andrivon 1998;Dorrance et al 1999;Vega-Sánchez et al 2000). These procedures may influence aggressiveness and virulence results (Zarzycka 1995(Zarzycka , 1996.…”

Host Diversity Can Reduce Potato Late Blight Severity for Focal and General Patterns of Primary Inoculum

“…Also, lesion expansion in late blight can be very important, especially for stem lesions that can expand to destroy a large portion of individuals. Overall levels of variability tend to be low in populations of the causal agent, Phytophthora infestans, because only one or a few clonal lineages are represented in many populations (3,9,11), so there may be little pathogen specialization to manipulate through host mixtures. The above predictors suggest that hostdiversity effects would be smaller for late blight of potatoes than for rusts and mildews of small grains.…”

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

“…The US-11 genotype originated as an especially fit recombinant of the US-6 and US-8 genotypes and displaced both the US-1 and US-6 genotypes. By 1997, US-8 and US-11 were the predominant P. infestans genotypes affecting potato and tomato crops (Dorrance et al 1999;Gavino et al 2000). 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).…”