Effect of Soil Temperature and Soil-Water Matric Potential on the Survival of Phytophthora capsici in Natural Soil. J. H. Bowers, Former Graduate Student, Department of Plant Pathology, Cook College-New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick 08903. G. C. Papavizas, and S. A. Johnston. Plant Pathologist, Biocontrol of Plant Diseases Laboratory, Plant Sciences Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; and Extension Specialist in Plant Pathology, New Jersey Agricultural Experiment Station, Rutgers University, Bridgeton 08302. Plant Dis. 74:771-777. Accepted for publication 20 March 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/PD-74-0771.

Field tests were established during the summer of 1982 and the winter of 1982–1983 to determine the ability of sporangia, zoospores, and oospores of Phytophthora capsici to survive at two depths, 4–6 and 20–22 cm. Average soil moisture and temperature were recorded daily. During the summer tests in a bioassay using soil sampled at 0 and 3 days, and 1, 2, 4, 6, 8, and 12 wk after initial infestation with sporangia of P. capsici, incidence of disease on pepper (Capsicum annuum) seedlings was 100% initially and declined until no disease was observed at 8 wk. As assayed in dilution plate tests, the number of propagules declined over time. In soil initially infested with zoospores, disease incidence in the bioassay declined until no disease was observed at 4 wk. Oospores, embedded in nylon mesh and placed in soil, were tested for viability with the vital stain tetrazolium bromide. Oospore viability decreased from 72.5% to 50–60% after 12 wk. Incidence of disease was low in a bioassay after 1 wk but increased to 100% after 12 wk. Depth in soil was not a significant factor for survival of any propagule (P = 0.05). In the overwintering test, oospore viability declined from 67% to 25–29% after 16 wk and to 8–9% after 27 wk. No disease was observed in the bioassay after 8 wk. In controlled environment tests designed to evaluate the effects of defined soil moisture and temperature levels on survival of the various propagules of P. capsici, all propagule types survived longer at –10² and –10³ kPa and 15 and 25 C than at –10 and –30 kPa and –5 and 35 C. Low survival at –10 kPa and –5 and 5 C may explain the low survival observed in the overwintering field test where the weather was wet and cold. However, oospores were viable and were able to cause disease in a bioassay after 16 wk in most of the controlled moisture-temperature combinations. Thus, oospores may be the overwintering propagules of P. capsici.