VIEW ARTICLE

Ecology and Epidemiology

Overwintering and Survival of Phytophthora parasitica, Causing Dieback of Rhododendron. C. R. Kuske, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Present address of senior author: Monsanto Agricultural Chemical Co., 800 N. Lindbergh, St. Louis, MO 63137; D. M. Benson, associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 73:1192-1196. Accepted for publication 24 March 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1192.

Two isolates of Phytophthora parasitica from rhododendron survived <3 days at temperatures below - 4.5 C, and <11 days at temperatures between 0 and - 4 C on a thermal gradient plate. At temperatures between 1 and 2 C, the fungus survived 21 to over 40 days in infected leaf disks depending on tissue collection date, and was recovered from 80- 100% of leaf disks after 40 days at temperatures above 4 C, Cold acclimation of colonized leaf disks for 3 days at 4 C before placement on the gradient plate had no effect on fungal tolerance to cold temperatures. In the winter of 1980- 1981. P. parasitica overwintered in infected leaf disks buried 5 cm deep in a layer of pine bark (called a container base) upon which potted nursery plants are placed, but not in infected leaf disks on the surface of the container base or on the surface of a pine bark medium in 2.4-L pots resting on the container base. The fungus survived in buried infected leaf disks during the 1981- 1982 winter as well. Low percentages of P. parasitica were recovered from infected main stems and roots of hybrid rhododendron plants and infected leaf disks on the surface of the container base in 1981- 1982. P. parasitica was recovered from the naturally infested pine bark from March through December, but not following periods of cold weather in January and February. Mycelium, chlamydospores, and sporangia of P. parasitica were observed in overwintered leaf disks that were buried 5 cm deep in the container base of pine bark during the 1981- 1982 winter season. Colonies of P. parasitica originated from chlamydospores in the leaf disks.