Thermal Sensitivity of Three Species of Phytophthora and the Effect of Soil Solarization on Their Survival. C. Juarez- Palacios, Department of Plant Pathology, University of California, Davis 95616. R. Felix-Gastelum, R. J. Wakeman, E. J. Paplomatas, and J. E. DeVay. Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 75:1160-1164. Accepted for publication 7 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1160.

Heat sensitivity and the effect of soil solarization on the survival of hyphae and spores of Phytophthora cinnamomi, P. cactorum, and P. megasperma were determined in inoculate walnut twigs and artificially infested Reiff silty clay loam soil. Viable chlamydospores of P. cinnamomi were not detected in infested soil exposed to 45 C for 20 min. In contrast, oospores of a high-temperature isolate of P. megasperma survived exposure for 30 min at 45 C, whereas P. cactorum was killed within 30 min at 45 C. In field studies, solarized soil reached a maximum temperature of 45 C at the 15-cm depth and 33 C at the 45-cm depth, compared with nonsolarized soil where maximum temperatures of 31 and 28 C were recorded at 15 and 45 cm, respectively. No activity of P. cinnamomi was detected in infested soil after 2 wk at the 30-cm depth or after 4 wk at the 45-cm depth in solarized soil, whereas there was little or no reduction in survival in nonsolarized soil. P. cactorum withstood the effects of soil solarization at the 30- and 45-cm depths but was killed within 2 wk at the 15-cm depth, with little or no change in the percentages of infection of peach leaf disks used as bait in nonsolarized soil. Some propagules of the high-temperature isolate of P. megasperma survived the treatment, although the percentage of leaf disk infection was greatly reduced after soil solarization at the 15-cm depth during 4 wk. The heat sensitivity of the isolates of Phytophthora in laboratory experiments closely reflected their inactivation in solarized soil and indicated the possible use of soil solarization for management of these pathogens.