Influence of Soil Temperature and Inoculum Density ofPhytophthora cinnamomion Root Rot of Fraser Fir

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Abstract
Known numbers of P. 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.degree., 16.degree., 19.degree., 22.degree. and 25.degree. C were studied. Inoculum densities of 10, 50, 100, 500, 1000 and 5000 chlamydospores/kg 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 16.degree.-25.degree. C. An ID50 value for infection (inoculum density required for infection of 50% of the total number of plants) of 322 chlamydospores/kg of soil was obtained when inoculum density effects were averaged over temperatures of 16.degree.-25.degree. C. Optimum temperatures for mortality of Fraser fir seedlings were 19.degree.-25.degree. C. Soil temperatures below 19.degree.-25.degree. C delayed the onset of foliar symptoms from an average of 16 days at 25.degree. C to 34 days at 16.degree. C. Although significant root infection took place at 12.degree. and 14.degree. C, foliar symptoms were not expressed by infected seedlings when incubated below 16.degree. C. The minimum level of P. cinnamomi inoculum in soil required for infection and mortality of Fraser fir seedlings was fewer than 10 chlamydospores/kg of soil.