Harry A. J. Hoitink and Steven T. Nameth, Professors, Department of Plant Pathology, Ohio Agricultural Research and Development Center, The Ohio State University; and James C. Locke, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, Wooster, Ohio.
The Life Cycle of Phytophthora Diseases
Phytophthora species associated with woody plants generally over-winter in infected plants or in decaying infested plant debris in the soil or container medium. They generally do not survive after infested crop residues have decomposed in soil. Most do not survive extremely cold winters in crop residue.
An exception is Phytophthora citrophthora, which causes a severe crown rot and shoot blight problem on Pieris japonica. It also causes dieback on rhododendron and root rot of Taxus. This species can survive in infested tissues on the surface of the soil even in winters with -25°F temperatures.
Irrigation water collected from streams or irrigation ponds typically is contaminated with Phytophthora spp. Water used for greenhouse crops should be disinfested but that is not practical for the large volumes of water used on containers. Slow sand filters have been developed recently that are very effective for treatment of irrigation water for greenhouse crops.
Phytophthora spores germinate in water droplets or in a film of water. Spores may be splashed with water droplets onto foliage or be moved in running water in the soil or on the soil surface. Some spores known as zoospores are motile and swim towards root tips or stomata on leaves where they cause infections. Leaves must be wet for at least two hours for infections to be successful. Dry leaves do not become infected even when the humidity is high.