Population Densities of Phytophthora capsici in Field Soils in Relation to Drip Irrigation, Rainfall, and Disease Incidence. Jean B. Ristaino, Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Melanie J. Hord, and Marcia L. Gumpertz. Former Graduate Research Assistant, Department of Plant Pathology, and Assistant Professor, Department of Statistics, North Carolina State University, Raleigh 27695. Plant Dis. 76:1017-1024. Accepted for publication 21 April 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1017.

Population dynamics of Phytophthora capsici were monitored in artificially infested plots in fields planted with bell peppers (Capsicum annuum). Plots were drip-irrigated on either a more frequent (three times per week) or a less frequent (one or two times after infestation) schedule and were infested with one of three levels of inoculum or left uninfested. Pathogen population densities increased and then decreased over time in each field. In a field with moderate rainfall, plots had population densities of 0, 5, 18, and 41 cfu/g of dry soil at the time of infestation. Population densities were higher 41 days after planting in plots irrigated more frequently than in those irrigated less frequently. Population densities early in epidemic development were positively correlated with disease incidence and soil water content at the time of sampling. In the moderate rainfall field, changes in population densities over time were greater on the side of the plots where the drip irrigation line was located than on the opposite side. Population densities in plots of a field with high rainfall were 0, 2, 8, and 38 cfu/g of dry soil at the time of infestation and were not affected significantly by the frequency of irrigation or the location of the drip irrigation line. Population densities in the high rainfall field were highest 42 days after planting following a heavy rainfall early in the season. The pathogen spread into uninfested plots and was detected after symptoms of disease appeared in each field. At the end of the season, the field with high rainfall had higher population densities of P. capsici in initially uninfested plots than the field with moderate rainfall. In both fields, population densities in soil were too high to allow determination of a threshold below which disease was negligible, and populations declined when disease incidence was high.