Survival of Phytophthora ramorum in Recirculating Irrigation Water and Subsequent Infection of Rhododendron and Viburnum

Phytophthora ramorum was studied in an open air simulation system with nine separate container stands each connected to its own water collection system. The water in these reservoirs was inoculated with P. ramorum and then used for overhead irrigation over the course of the season to study the spread of the pathogen and development of P. ramorum blight in Rhododendron and Viburnum spp. P. ramorum could infect plants through the use of contaminated irrigation water, with the maximum amount of infection of Rhododendron spp. less than 19%. In the 2 years of the study, symptom onset occurred 8 and 16 days, respectively, after water was first inoculated. The disease rate proportion of infected plants developing symptoms varied with year and season. In both years, the pathogen was detected in the water reservoirs over the course of the growing season.