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Ecology and Epidemiology

Relationship Between Inoculum Level of Phytophthora capsici and Mortality of Pepper. J. H. Bowers, Department of Plant Pathology, University of Florida, Gainesville 32611, Present address: Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691; D. J. Mitchell, Department of Plant Pathology, University of Florida, Gainesville 32611. Phytopathology 81:178-184. Accepted for publication 22 August 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-178.

Relationships between the levels of oospore and zoospore inoculum of Phytophthora capsici and mortality of pepper (Capsicum annuum) were investigated. Oospore progeny from 20 pairings of pathogenic isolates of P. capsici varied in pathogenicity to pepper at an initial inoculum density of 25 oospores per gram of soil. Only oospore inoculum from specific pairings caused disease in growth room experiments. The highest mortality (30–75%) resulted from oospore inoculum produced from pairings involving certain isolates of A2 compatibility type. Oospore inoculum from other pairings caused little or no disease (0–5%). Further tests with oospores produced from one pairing resulted in an increase in disease with an increase in inoculum density. The ID₅₀ was calculated to be 41 oospores per gram of soil. Inoculum efficiency, defined as the estimated number of infections per propagule and calculated as the slope of the regression line when the estimated number of infections (via the multiple infection transformation) was regressed over inoculum density, was 0.011. Low numbers of zoospores applied to the base of an expanding pepper leaf were capable of causing high levels of plant mortality; 75–95% plant mortality occurred with three zoospores per plant. When zoospores were added to free water above flooded soil, 75 and 95% of the plants exposed to 10 and 25 zoospores per plant, respectively, died in some experiments.