Virulence of Five 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, was considered the main causal agent of the disease. However, a recent survey of soilborne species from symptomatic rhododendrons in Oregon revealed that is more common than , and that several other species may be involved. We investigated the ability of the five most abundant species from the survey to cause root rot: , , , , and . Three to four isolates were selected for each species from across six Oregon nurseries. Media of containerized 'Boursault' was infested with single isolates in a randomized complete block design in a greenhouse. , and rapidly caused ≥90% of severe root rot, whereas caused more moderate disease (46% of severe root rot). 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 that caused less disease than other isolates. This study demonstrates that all five 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.