Influence of Soil Temperature and Inoculum Density of Phytophthora cinnamomi on Root Rot of Fraser Fir. H. D. Shew, Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. D. M. Benson, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Plant Dis. 67:522-524. Accepted for publication 30 September 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-522.

Known numbers of Phytophthora cinnamomi chlamydospores were added to a sandy loam soil maintained in 10-cm-diameter pots in constant temperature water baths in the greenhouse. Soil temperatures of 14, 16, 19, 22, and 25 C were studied. Inoculum densities of 10, 50, 100, 500, 1,000, and 5,000 chlamydospores per kilogram of soil resulted in 3, 10, 22, 60, 75, and 100% infection of Fraser fir seedlings, respectively, at optimum temperatures for disease development. Optimum temperatures for infection were between 16 and 25 C. An ID₅₀ value for infection (inoculum density required for infection of 50% of the total number of plants) of 322 chlamydospores per kilogram of soil was obtained when inoculum density effects were averaged over temperatures of 16–25 C. Optimum temperatures for mortality of Fraser fir seedlings were between 19 and 25 C. Soil temperatures below 19–25 C delayed the onset of foliar symptoms from an average of 16 days at 25 C to 34 days at 16 C. Although significant root infection took place at 12 and 14 C, foliar symptoms were not expressed by infected seedlings when incubated below 16 C. The minimum level of P. cinnamomi inoculum in soil required for infection and mortality of Fraser fir seedlings was determined to be fewer than 10 chlamydospores per kilogram of soil.