Characterization of Isolates of Phytophthora infestans from Tomato and Potato in North Carolina from 1993 to 1995

Abstract

Eighty-five isolates of Phytophthora infestans from 33 tomato and 8 potato fields in North Carolina, South Carolina, and Tennessee were collected from 1993 to 1995 and tested for mating type, sensitivity to metalaxyl, and allozyme genotype at glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) loci. One isolate collected from potato in eastern North Carolina in 1994 was the A1 mating type, whereas all other isolates from potato and tomato were the A2 mating type. Six isolates were sensitive to metalaxyl (<40% growth at 1.0 μg of metalaxyl per ml), nineteen isolates were intermediate in sensitivity to metalaxyl (>40% growth at 1.0 μg of metalaxyl per ml and <40% growth at 100 μg of metalaxyl per ml), and sixty isolates were resistant to metalaxyl (<40% growth at 1.0 and 100 μg of metalaxyl per ml). Four different allozyme genotypes at the Gpi and Pep loci were identified. The single A1 isolate found on potato in eastern North Carolina had the dilocus allozyme genotype Gpi 86/100, Pep 92/100 and was identified as the US-1 genotype. Fifty-five isolates had the dilocus allozyme genotype Gpi 100/111, Pep 100/100 and were classified as the US-7 genotype, whereas twenty-four isolates were Gpi 100/111/122, Pep 100/100 and were classified as the US-8 genotype. Two isolates that were sensitive to meta-laxyl and seventeen isolates that were intermediate in sensitivity to metalaxyl were found among the US-7 and US-8 genotypes. In addition, five isolates had the allozyme genotype Gpi 100/100, Pep 92/100 (similar to the previously reported US-6 genotype), but they were the A2 mating type and either sensitive or intermediate in response to metalaxyl. These isolates composed a new genotype not previously reported in the United States and were designated as US-18. The US-7 genotype was more frequent on tomato in western North Carolina and the US-8 genotype was present on potato in eastern North Carolina, indicating that different inoculum sources are responsible for epidemics on the two crops in different regions of the state.

Population studies on Phytophthora infestans on potatoes and tomatoes in southern Germany

“…The fact that metalaxyl resistance was found also on untreated fields agrees very well with findings of other authors (Fraser et al 1999;Day et al 2004). Our own data suggest that a high rate of migration of resistant isolates into unsprayed fields took place, as reported previously by other authors (Fraser et al 1999).…”

“…Our own data suggest that a high rate of migration of resistant isolates into unsprayed fields took place, as reported previously by other authors (Fraser et al 1999). Fraser et al (1999) found that the frequency of metalaxyl-resistant isolates was not significantly greater in metalaxyl-sprayed than in unsprayed fields.…”

“…Our own data suggest that a high rate of migration of resistant isolates into unsprayed fields took place, as reported previously by other authors (Fraser et al 1999). Fraser et al (1999) found that the frequency of metalaxyl-resistant isolates was not significantly greater in metalaxyl-sprayed than in unsprayed fields. Other authors have shown that the application of metalaxyl can select for individuals in a population with increased insensitivity to the fungicide (Daggett et al 1993).…”

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

“…The P. infestans US-7 genotype was the predominant in the Columbia Basin of Washington and Oregon during 1993, and was highly pathogenic on tomatoes in addition to potatoes ). The P. infestans US-8 genotype was predominant in most other potato-growing areas of Canada and the United States since the mid-1990s and displaced all other genotypes (Fraser et al 1999;Fry and Goodwin 1997a). The P. infestans US-8 genotype was reported to have greater pathogenic fitness on the potato, with higher infection efficiency, sporulation capacity, and shorter latent period on potato foliage and tubers than other genotypes (Lambert and Currier 1997;Miller et al 1998).…”

“…This was evident from the appearance, in the late 1970s, of metalaxyl-resistant isolates of P. infestans in European populations after only a few seasons of fungicide application (Dowley and O'Sullivan 1981). By 1993, the metalaxyl-resistant isolates of the US-7 and US-8 genotypes had displaced the metalaxyl-sensitive isolate of the US-6 genotype (Fraser et al 1999). The P. infestans US-7 genotype was the predominant in the Columbia Basin of Washington and Oregon during 1993, and was highly pathogenic on tomatoes in addition to potatoes ).…”