SPATIAL-DISTRIBUTION OF PHYTOPHTHORA-CACTORUM IN NEW-YORK APPLE ORCHARD SOILS

Population assessments of dormant spores of Phytophthora cactorum in a pple orchard soils yielded three clear distribution gradients. Populat ions at the bottoms of slopes were relatively high, declined with incr easing 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. The re 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, propag ule numbers declined significantly after 18 months at or near the soil surface, but remained constant at 7- to 10-cm depth, indicating conti nual renewal of surface populations to maintain the steep depth gradie nt. Fallen apple leaves, fruit, and petals were all naturally colonize d by P. cactorum in the field. Surface amendments with inoculated leav es 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/m (2) from May to September) were collected from the soil surface beneat h apple trees. These contained relatively high populations of P. cacto rum at densities comparable with those in the surface layers of soil a nd were likely to have contributed to the steep vertical gradient obse rved.