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Identification of a New Phytophthora Species Causing Root and Runner Rot of Cranberry in New Jersey

In New Jersey, Phytophthora cinnamomi is the pathogen most commonly isolated from diseased roots and runners of the cultivated cranberry (Vaccinium macrocarpon). A second distinct species of Phytophthora has been isolated from dying cranberry plants and surface irrigation water. This species is homothallic with paragynous antheridia and ellipsoid-limoniform, nonpapillate sporangia. It was tentatively identified as P. megasperma in an earlier report. Laboratory experiments demonstrate that the cardinal temperatures for vegetative growth are between 5 and 30°C with an optimum near 25°C. Sporangia are produced at temperatures between 10 and 20°C with the majority of sporangia produced at 10 and 15°C. In pathogenicity tests, no growth effect was observed on cranberry plants (cv. Early Black) when tests were conducted at 25°C; however, significant reductions in plant growth occurred when tests were conducted at 15°C. This species was insensitive to metalaxyl but was sensitive to buffered phosphorous acid. Sequence analysis of the internal transcribed spacer 1 (ITS1), 5.8S rDNA, and ITS2 regions place these isolates in Phytophthora clade 6 with greatest similarity to Phytophthora taxon raspberry. To our knowledge, this is the first report of isolates of this affiliation in North America. However, the observation of low temperature preferences makes this species unique in an otherwise high temperature clade. The isolates described in this study are tentatively classified as Phytophthora taxon cranberry.