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Enhancing Detection of Phytophthora cactorum in Naturally Infested Soil. S. N. Jeffers, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456, Present address: Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706; H. S. Aldwinckle, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Phytopathology 77:1475-1482. Accepted for publication 13 April 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1475.

Reliable methods were needed to detect Phytophthora cactorum, one of the primary causal agents of Phytophthora crown rot of apple trees, in its natural soil environment. Apple or pear fruits, used in a baiting bioassay, were ineffective at detecting P. cactorum in naturally infested soil. Apple seedlings, cotyledons, and seedling leaf pieces were successful baits, but cotyledons were the most sensitive and efficient. Completely air drying soil subsamples and then remoistening them for several days before flooding and adding plant tissue baits (extended baiting procedure) greatly enhanced the detection of P. cactorum when compared to the standard, direct baiting procedure without prior manipulation of soil moisture. Bioassay incubation temperature, volume of water added to remoisten air-dried soil subsamples, and incubation period following remoistening all affected detection, but the photoperiod during incubation did not. The advantages of an extended baiting bioassay with apple cotyledons were: greater sensitivity than with pear or apple fruits or by direct baiting, readily available and inexpensive baits, formation of sporangia of P. cactorum directly on necrotic cotyledons, and lack of interference by contaminating Pythium species. Cotyledons also were colonized by zoospores of P. cambivora, P. citricola, and P. cryptogea, but not by those of P. megasperma, P. syringae, P. drechsleri, or an unidentified Phytophthora species. The extended bioassay procedure routinely has provided a relatively rapid and efficient means of detecting P. cactorum in a diversity of soils within and around New York apple orchards.