PhytophthoraandPythiumspecies Associated with Crown Rot in New York Apple Orchards

Abstract

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 [penta chloronitrobenzene] medium. The most frequently isolated species was Phytophothora megasperma, which was recovered from 8 trees. Phytophthora cactorum, generally regarded as the causal organisn, was recovered from 3 trees. Other pythiaceous fungi, including Pythium irregulare, 2 unidentified isolates of Phytophthora, and 5 unidentified isolates of Pythium, were each recovered from only 1 tree. Relative pathogenicity of 3 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 P. cactorum isolates were most pathogenic in both assays. All tested isolates of P. megasperma were consistently pathogenic, implicating this species for the 1st time in crown rot of apple trees in New York. P. negasperma isolates, like those of P. cactorum, exhibited varying degrees of virulence to specific apple cultivars.