Effects of Temperature, Concentration, Age, and Algaecides on Phytophthora capsici Zoospore Infectivity.

Abstract

Controlled laboratory studies were undertaken to determine the effects of water temperature (2, 9, 12, 19, 22, and 32°C), inoculum concentration (1 × 102, 1 × 103, 5 × 103, 1 × 104, 2 × 104, and 4 × 104 zoospores/ml), and zoospore suspension age (0, 1, 3, and 5 days old) on infection of pickling cucumbers (Cucumis sativus) by Phytophthora capsici. Zoospore motility and mortality in response to commercial algaecides were also investigated. Cucumbers became infected at all temperatures tested, except 2°C, and the highest infection incidence was observed for cucumbers incubated in suspensions held at ≥19°C. Fewer fruit (<40% at ≥19°C, 0% at ≤12°C) became infected when water contained 1 × 102 zoospores/ml. Almost 100% of fruit were infected when water contained ≥5 × 103 zoospores/ml at temperatures ≥12°C. While the incidence of fruit infection declined with the zoospore suspension age, infection still occurred when 5-day-old suspensions were used. Commercial algaecides inhibited zoospore motility and caused significant zoospore mortality in laboratory assays, and show promise for treatment of infested irrigation water. Avoidance of infested irrigation water throughout the growing season is warranted until effective and economically acceptable water treatments are developed for field use.