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Relationships of Inoculum Levels of Several Soilborne Species of Phytophthora and Pythium to Infection of Several Hosts. D. J. Mitchell, Associate Professor, Department of Plant Pathology, University of Florida, Gainesville, FL 32611; Phytopathology 68:1754-1759. Accepted for publication 16 June 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1754.

The relationships of inoculum densities of several species of Phytophthora (P.) and Pythium (Py.) to percentages of infection of various hosts were determined by exposing roots to chlamydospores or oospores uniformly mixed with soil or by adding zoospores to flooded soil around plants. Between 15 and 43 oospores per gram of soil were required for 50% infection of peanut, rye, or soybean by Py. myriotylum; amaranthus, cotton, or tomato by Py. aphanidermatum; or of cabbage by Py. polymastum. With host-pathogen combinations of papaya-P. palmivora and milkweed vine-P. citrophthora, 0.2 to 0.9 chlamydospores per gram of soil were required for 50% infection. Two hundred and fifty, 276, or 281 zoospores of Py. aphanidermatum, P. cryptogea, Py. ostracodes per plant were required for 50% infection of tomato, watercress, or cotton, respectively. Slopes of regression lines of log10 logc [(1/1-x)], where x equals the percentage of infected plants, on log10 inoculum density were 0.62 to 0.70 with oospores of Pythium spp., 0.69 with zoospores of Py. aphanidermatum or P. cryptogea, and approached 1.0 with chlamydospores of Phytophthora spp. Inoculum density increased in soil infested with chlamydospores, but not in soil infested with zoospores, through the production of secondary propagules in the vicinity of plant roots.

Additional keywords: epidemiology, Amaranthus tricolor, Arachis hypogaea, Brassica oleracea, Carica papaya, Glycine max, Gossypium hirsutum, Lycopersicon esculentum, Morrenia odorata, and Nasturtium officinale.