VIEW ARTICLE

Ecology and Epidemiology

Phytophthora citrophthora on Pieris japonica: Infection, Sporulation, and Dissemination. W. W. P. Gerlach, Graduate Research Associate, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691, Present address of senior author: Standard (Philippines) Fruit Corp., P. O. Box 362, MAKATI, Rizal, Philippines; H. A. J. Hoitink(2), and A. F. Schmitthenner(3). (2)(3) Associate Professor, and Professor, respectively The Ohio State University, Columbus 43210. Phytopathology 66:302-308. Accepted for publication 26 August 1975. DOI: 10.1094/Phyto-66-302.

Young leaves and twigs of Pieris japonica are most susceptible to Phytophthora citrophthora. Mature leaves are resistant, except when wounded. The minimum time required for penetration of leaves (zoospore inoculum) was 15, 2, and 4 hours at incubation temperatures of 12, 24, and 32 C, respectively. Greatest accumulation of zoospores occurred in the dark at fully developed stomata on the lower surface of young leaves. Abscission of leaves of intermediate maturity was affected by the distance between the lesion and the petiole. Sporangia were produced on wet abscised leaves in light at 8-32 C, but most rapidly at 24 C. Very few sporangia formed on initially wetted leaves that had dried to less than 32% of fresh weight. A negative disease dispersal gradient (b = –8.3) was obtained for the distribution of lesions in the field, indicating that inoculum was splashed up from the ground. Zoospores released from sporangia on abscised leaves also invaded meristematic tissues of axillary buds at the crown and feeder roots, and caused crown and root rot.