Survival and Splash Dispersal ofPhytophthora parasitica,Causing Dieback of Rhododendron

Abstract

P. parasitica survived in air-dried, rhododendron leaves for 1-2 h. Survival rate rapidly declined after 1-3 days and the fungus could not be recovered after 18-22 days. Sporangium production on leaf disks that had been air-dried prior to placing in water was reduced to 0.2-21% of that of nondried leaf disks. Zoospores were not released from infected leaf disks that were air-dried for 3 h or more and then flooded. Sporangia and zoospores were formed on infected leaf disks held at a high relative humidity (RH). Maximum zoospore production occurred 24-48 h in flooded conditions. In nursery studies, the initial incidence of lesions induced on rhododendrons by Phytophthora was affected by weather conditions. Even though plants were irrigated daily, rainfall was significantly correlated with lesion incidence from June to mid-Sept. and lesion numbers were greatest after periods of rain preceded by days with several hours of RH .gtoreq. 90%. Incidence of initial lesion induction by Phytophthora on 1- and 2-yr-old plants grown in pots resting on a naturally infested layer of pine bark (called a container base) followed a negative dispersal gradient with height from the base, indicating that propagules of Phytophthora were splashed up from the container base onto the leaves of the plants.