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Spatial Distribution of Phytophthora cactorum in New York Apple Orchard Soils. I. J. Horner, Horticulture and Food Research Institute of New Zealand, Private Bag 92-169, Auckland; W. F. Wilcox, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 86:1122-1132. Accepted for publication 19 June 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-1122.

Population assessments of dormant spores of Phytophthora cactorum in apple orchard soils yielded three clear distribution gradients. Populations at the bottoms of slopes were relatively high, declined with increasing distance up slopes, and strongly correlated with soil moisture content. Populations decreased with increasing distance from the tree trunk, becoming close to nil outside the tree-row herbicide strip. There was also a sharp decline in P. cactorum populations with increasing depth with approximately 50 and 70% of propagules in the top 3 and 6 cm of soil, respectively. In the absence of organic substrates, propagule numbers declined significantly after 18 months at or near the soil surface, but remained constant at 7- to 10-cm depth, indicating continual renewal of surface populations to maintain the steep depth gradient. Fallen apple leaves, fruit, and petals were all naturally colonized by P. cactorum in the field. Surface amendments with inoculated leaves in the fall resulted in a substantial increase in soil populations measured the following spring, both in microplots and directly beneath mature apple trees. Large quantities of earthworm castings (1.45 kg/m2 from May to September) were collected from the soil surface beneath apple trees. These contained relatively high populations of P. cactorum at densities comparable with those in the surface layers of soil and were likely to have contributed to the steep vertical gradient observed.

Additional keywords: epidemiology, oospores, quantification, SADAMCAP.