VIEW ARTICLE

Ecology and Epidemiology

Temporal Changes in Activity and Dormant Spore Populations 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 86:1133-1139. Accepted for publication 19 June 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-1133.

Dormant spore populations of Phytophthora cactorum in three New York apple orchard soils were measured over a 2-year period using the SADAMCAP (soil air-dried and moistened chilled and plated) technique. At all three sites, populations were highest in early spring, declined steadily throughout the summer and fall, and increased again the following spring. Activity, defined as the presence of germinable sporangia or zoospores in the soil, was assessed and quantified using the “brief baiting” and SCAP (soil chilled and plated) techniques. For brief baiting, soil samples were flooded and baited with apple cotyledons for 4 h, and then baits were removed and further incubated in distilled water to determine colonization. For SCAP, soil samples were flooded and chilled, the drained water was plated on selective agar, and P. cactorum colonies were enumerated. There was no activity in field samples collected from late November to March. The first activity in spring coincided with the time that soil temperatures exceeded 10ēC. In laboratory studies, there was no colonization of cotyledon baits when air-dried soil naturally infested with P. cactorum was remoistened, flooded, and baited at temperatures below 10ēC. However, following incubation of soil at 22ēC to allow oospore germination and sporangial formation, cotyledons were infected at temperatures down to 7ēC. Once cotyledons were colonized, the fungus sporulated on leaf margins down to 6ēC. In apple orchards, there was no correlation between measured P. cactorum activity and soil temperature when only periods above 10ēC were considered. Neither activity nor oospore populations were closely associated with rainfall over the previous 5 days.