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Metalaxyl Insensitivity of Phylophthora erythroaeptica Isolates Causing Pink Rot of Potato in Maine. D. H. Lambert, Department of Plant Biology and Pathology University of Maine, Orono 04469. B. Salas, Department of Entomology, University of Maine, Orono 04469. Plant Dis. 78:1010. Accepted for publication 7 June 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1010B.

Potato (Solanum tuberosum L.) fields in Aroostook County, Maine were surveyed for Phytophthora tuber rot pathogens during September and October 1993. Although the majority of isolates were Phylophthora infestans (Mont.) de Bary, isolates causing pink rot were recovered from six separate fields. Radial growth rates of these strains were determined on metalaxyl (Ridomil 2E)-amended V8 agar incubated in the dark al 21 C for 6 days. Three Phytophlhora erythroseptica Pethybr. isolates had ED50 values greater than 320 ?g a.i./ml. The three other isolates, two of which were P. erythroseptica, had ED50 values less than or equal to 0.1 ?g/ml, typical of sensitive strains in other Phylophthora species. Whole Katahdin tubers were inoculated with single mycelium plugs, incubated in the dark at 21 C, and halved after 7 days. Symptomatic cross-sectional areas of the five P. erythroseptica isolates did not differ significantly (P = 0.05). In Maine, metalaxyl has been used for Pythium leak and Phytophthora late blight control since the early 1980s, typically with two applications per year in alternating years. Metalaxyl Insensitivity has been detected less frequently in soilborne than in aerial pathogen populations. This is the first report of Insensitivity to metalaxyl in P. erythroseptica. It was not detected in the western United States in a previous study (1).

Reference: (1) RW. Slack et al. Phytopathology 83:886, 1993.