Virulence of Five Phytophthora Species Causing Rhododendron Root Rot in Oregon

Phytophthora root rot is a destructive disease of rhododendron that causes substantial losses of this nursery crop in infested field and container production areas. Historically, Phytophthora cinnamomi was considered the main causal agent of the disease. However, a recent survey of soilborne Phytophthora species from symptomatic rhododendrons in Oregon revealed that P. plurivora is more common than P. cinnamomi, and that several other Phytophthora species may be involved. We investigated the ability of the five most abundant species from the survey to cause root rot: P. plurivora, P. cinnamomi, P. pini, P. pseudocryptogea, and P. cambivora. Three to four isolates were selected for each species from across six Oregon nurseries. Media of containerized Rhododendron catawbiense 'Boursault' was infested with single isolates in a randomized complete block design in a greenhouse. Phytophthora cinnamomi, P. pini, and P. plurivora rapidly caused ≥90% of severe root rot, whereas P. pseudocryptogea caused more moderate disease (46% of severe root rot). Phytophthora cambivora failed to produce enough inoculum and was used at a lower inoculum density than the other four species; however, occasionally, it caused severe root rot (5% incidence). No differences in virulence were observed among isolates of the same species, except for one isolate of P. plurivora that caused less disease than other P. plurivora isolates. This study demonstrates that all five Phytophthora species, which were representative of 94% of the survey isolates, are capable of causing severe root rot and plant death, but that not all species are equally virulent.