Overwintering and Survival ofPhytophthora cinnamomiin Fraser Fir and Cover Cropped Nursery Beds in North Carolina

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Abstract
P. cinnamomi survived extended periods of soil temperatures at or below 0% C for 2 consecutive winters as chlamydospores free in the soil, in pieces of organic matter, or in roots of symptomatic and asymptomatic Fraser fir (A. fraseri) seedlings. Inoculum densities decreased markedly in Dec. and Jan. as soil temperatures fell below 0.degree. C. Decreasing inoculum densities were highly correlated (P = 0.01) with decreasing soil temperatures. Soil matric potential (when > -0.45 bars), soil texture, initial inoculum density, or seedling age were not correlated with decreasing inoculum densities. Propagules of P. cinnamomi were recovered at every sampling date (Oct.-June) from nonrhizosphere soil in plots with Fraser fir, crimson clover [Tifolium incarnatum], rye [Secale cereale] or left fallow and from rhizosphere soil of crimson clover and rye plants. Chlamydospores were the most frequent form of P. cinnamomi isolated from the soil or root tissue during the winter and spring months. Survival of P. cinnamomi in the roots of asymptomatic seedlings demonstrated that foliar symptom expression is not a reliable indicator of the incidence of infection within a nursery bed.