Mycelium of P. cinnamomi in corn meal agar and chlamydospores in naturally infested soil were inactivated on a thermal gradient plate at temperatures below 0.degree. C. Rate of inactivation was directly related to number of degrees below 0.degree. C. Inactivation of mycelium of P. cinnamomi occurred in 2, 6 or 16 days, at -6.7, -3.8 and 1.4.degree. C, respectively. In a sandy-loam soil, chlamydospores of P. cinnamomi were inactivated after 2, 17, or 29 days at -6.4, -3.4 and -0.5.degree. C, respectively. Acclimatization of agar cultures or infested soil for 5-7 days at 4.degree. C did not increase tolerance of P. cinnamomi to subzero temperatures. Inoculum of P. cinnamomi in infested root segments of Abies fraseri, in colonized oat grains, or in naturally infested soil, was inactivated during winters (1976-1977 and 1977-1978) when soil temperature at a depth of 10 cm dropped to 0.degree. C or below, but not during the winter of 1975-1976 when soil temperatures remained ahove 0.degree. C.