VIEW ARTICLE

Ecology and Epidemiology

Mechanisms of Survival of Zoospores of Phytophthora parasitica in Irrigation Water. S. V. Thomson, Assistant Professor, Department of Plant Pathology, University of California, Berkeley 94720; R. M. Allen, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 66:1198-1202. Accepted for publication 24 March 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1198.

Zoospores of Phytophthora parasitica were present in water placed on field soils from foot-rot infested citrus groves within 10 minutes, and citrus leaves in the water were infected within the same time. Zoospores retained motility in irrigation water up to 20 hours at 20 C but at higher temperatures the period of motility was shorter. Zoospores encysted rapidly after the addition of nutrients, orange peel, or citrus leaves. Zoospore cysts germinated in sterile distilled water (SDW) and sterile and nonsterile irrigation waste water. When growth ceased, the protoplasm contracted within the hyphae. Empty cysts or hyphae often lysed, but intact hyphal fragments remained viable for 40 days at 25 C in untreated waste water and resumed growth after the addition of supplemental nutrients. At concentrations of 10-1,000 mg glucose/liter of SDW plus 100 mg CaCl₂ or in the presence of orange peel or citrus leaves, hyphal tips in contact with container surfaces often produced appressoriumlike structures which usually produced microsporangia when nutrients were replaced with autoclaved or nonsterile irrigation waste water. Microsporangia could persist in nonsterile irrigation waste water at 24 C for 60 days, but most germinated sooner, releasing only one zoospore. Since mycelial inoculum from these zoospores was pathogenic to roots of citrus seedlings, zoospores of P. parasitica or structures produced by them probably play a significant role as survival or dispersal units in recycled water.