Response of Phytophthora spp. to Metalaxyl in Forest Tree Nurseries in the Pacific Northwest. P. B. Hamm, Research Assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. S. J. Cooley, Plant Pathologist, USDA, Forest Pest Management, P.O. Box 3623, Portland, OR 97208; and E. M. Hansen, Associate Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Plant Dis. 68:671-673. Accepted for publication 9 April 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-671.

Effects of metalaxyl on composition of Phytophthora spp., survival of Phytophthora in roots and soil, and severity of root disease of Douglas-fir were investigated at two commercial forest tree nurseries. One application suppressed root rot. Of three Phytophthora spp. isolated from treated seedlings (P. megasperma, P. drechsleri, and P. pseudotsugae), only P. pseudotsugae decreased in isolation frequency because of the chemical. Survival of Phytophthora spp. in infected seedlings remained high after treatment. At one nursery, 10 mo after the first application, Phytophthora spp. were isolated from 92% of the seedlings across fungicide treatments, whereas at the second nursery, isolation frequencies from seedlings were 77, 70, 29, and 13%, respectively, after zero, one, two, and three applications. Phytophthora was recovered from previously healthy seedlings 8 wk after they were transplanted into naturally infested, metalaxyl-treated soil.