Phytophthora pini Leonian, recently re-established from P. citricola I, is a pathogen with a wide range of forest and nursery hosts. It causes foliar infections in horticultural nurseries in Oregon, where recirculating irrigation systems are common. Increased use of recirculating irrigation systems may contribute to disease caused by waterborne plant pathogens. Simulated nursery chamber experiments were utilized to investigate the relationship between Phytophthora pini zoospore inoculum dose and disease on Rhododendron. Disease incidence in this system was unexpectedly low despite high inoculum levels tested, so further experiments under lab conditions were conducted to explore possible causes. Detached leaf assays were conducted to determine how inoculum dose, leaf wounding, and agitation of zoospore inoculum affected foliar infection of Rhododendron. Wounded and nonwounded leaves were dipped into suspensions of zoospores that were either untreated, mechanically agitated by vortexing to cause encystment, or pumped through an irrigation sprayer system. Disease severity (lesion area) and incidence (number of lesions per leaf area) were measured over seven days. At inoculum levels of ���10,000 propagules/mL, motile zoospores infected both wounded and nonwounded leaves. Vortexing or pumping resulted in zoospore encystment, and inoculation with these treatments caused disease almost exclusively on wounded leaves. No disease symptoms were observed following inoculation with any inocula at ��� 2,000 propagules/mL. Scanning electron microscopy of leaves inoculated with encysted propagules showed germinated cysts with hyphae growing over and around stomata without entering leaf tissue until reaching a wound site. Nonwounded leaves inoculated with motile spores showed stomata penetrated by hyphae. These findings indicate the importance of zoospore motility in reaching suitable infection sites, and demonstrate the impact of zoospore encystment on disease development. This has implications for disease management in nurseries where pruning wounds are common and the pumping of infested irrigation water may influence zoospore motility and infectivity. / Graduation date: 2013