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Seasonal Variation in Extent of Colonization of Two Apple Rootstocks by Five Species of Phytophthora. S. N. Jeffers, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. H. S. Aldwinckle, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Plant Dis. 70:941-945. Accepted for publication 12 May 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-941.

Two apple rootstock clones, MM.111 (field resistant to Phytophthora crown rot under New York orchard conditions) and MM.106 (field susceptible), were inoculated on the first of each month, except August, for 25 mo (January 1983 through January 1985). Previous growing seasonís shoots were cut into 65-mm lengths and inoculated in the laboratory with two isolates of Phytophthora cactorum, three isolates of P. megasperma, two isolates of P. cryptogea (A1), one isolate of P. cambivora (A1), and one isolate of an unidentified Phytophthora sp. (A1). All isolates had previously been recovered from diseased apple trees in New York. Necrosis values, calculated from the arcsin [square root] transformation of the proportion of twig length that was necrotic, were plotted over time to produce patterns of seasonal variation in the extent of colonization by Phytophthora spp. Over all 25 mo, MM.111 was colonized as much as or to a greater extent than MM.106 by all isolates except one of P. cactorum. There was a significant isolate ◊ rootstock interaction, which indicated a difference in virulence of these nine isolates to the two rootstocks. Monthly changes in the extent of colonization of both rootstocks by each isolate were highly significant; seasonal patterns between years were similar. The five species of Phytophthora fell into two groups depending on when relative peaks of colonization of the two rootstocks occurred: P. cactorum and P. cambivora had one peak during late spring and summer; P. megasperma, P. cryptogea, and Phytophthora sp. had two peaks, one during summer and one during winter.