VIEW ARTICLE

Etiology

Phytophthora and Pythium species Associated with Crown Rot in New York Apple Orchards. S. N. Jeffers, Graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; H. S. Aldwinckle(2), T. J. Burr(3), and P. A. Arneson(4). (2)(3)Associate professor and assistant professor, respectively, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; (4)Associate professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:533-538. Accepted for publication 13 August 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-533.

During the summers of 1978 and 1979, isolations were made from 23 apple trees showing typical crown rot symptoms in 10 western New York orchards. Two species of Phytophthora and one of Pythium plus other unidentified isolates of Phytophthora and Pythium were recovered on a pimaricin-vancomycin-PCNB medium. The most frequently isolated species was Phytophthora megasperma, which was recovered from eight trees. Phytophthora cactorum, generally regarded as the causal organism, was recovered from three trees. Other pythiaceous fungi, including Pythium irregulare, two unidentified isolates of Phytophthora, and five unidentified isolates of Pythium, were each recovered from only one tree. Relative pathogenicity of these isolates was determined in vitro by using an excised twig assay and in vivo by using seedlings grown in artificially infested soil. All species were pathogenic to some extent, but Ph. cactorum isolates were most pathogenic in both assays. All tested isolates of Ph. megasperma were consistently pathogenic, implicating this species for the first time in crown rot of apple trees in New York. Ph. megasperma isolates, like those of Ph. cactorum, exhibited varying degrees of virulence to specific apple cultivars.