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Phytophthora Crown Rot of Apple Trees: Sources of Phytophthora cactorum and P. cambivora as Primary Inoculum. 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 53706; H. S. Aldwinckle, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Phytopathology 78:328-335. Accepted for publication 12 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-328.

A recently developed baiting bioassay that enhanced the detection of Phytophthora cactorum was employed to determine the occurrence and distribution of this pathogen on apple nursery stock and in soils from within and around apple orchards. The bioassay also proved to be a sensitive means of detecting P. cambivora, a known crown rot pathogen, and P. citricola. Over a 2-yr period, 152 of 153 bundles of unbudded clonal rootstocks were found to be infested, 150 with P. cactorum and 111 with P. cambivora; 25 of 29 samples from unbudded seedling rootstocks were infested with only P. cactorum; and 158 of 175 bundles of nursery-grown trees on clonal rootstocks were infested, 158 with P. cactorum and only nine with P. cambivora. The incidences of P. cactorum and P. cambivora on 80 individual unbudded rootstocks from four clones were 76 and 73, respectively. All rootstock cultivars, with the possible exception of those grown from seed, were equally infested. All nurseries sampled in the United States, Canada, and Europe had contaminated plants. Naturally occurring inoculum on roots of unbudded rootstocks of Malling-Merton 106 apple caused severe root and crown rot when plants were flooded periodically. P. cactorum was detected in 56 of 112 soil samples collected around both symptomatic and healthy-appearing trees in 22 of 36 New York apple orchards. P. cactorum also was detected in 17 of 37 nonagricultural soils collected from sites in the vicinity of apple orchards. P. citricola frequently was present in these soils also. Isolates of P. cactorum recovered from soil were as virulent as those previously isolated from infected apple roots and crowns. Both infested nursery stock and infested orchard soils are potential sources of primary inoculum for Phytophthora crown rot of apple trees.

Additional keywords: Malus pumila, Phytophthora megasperma, Phytophthora syringae, soilborne plant pathogens.