Virulence of five phytophthora species causing rhododendron root rot in Oregon

Authors: SACHER, GABRIEL - Oregon State University; Scagel, Carolyn; Davis, Elizabeth; Beck, Bryan; Weiland, Gerald - Jerry

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2021
Publication Date: 1/24/2021
Citation: Sacher, G.O., Scagel, C.F., Davis, E.A., Beck, B.R., Weiland, G.E. 2021. Virulence of five phytophthora species causing rhododendron root rot in Oregon. Plant Disease. https://doi.org/10.1094/PDIS-09-20-1873-RE.
DOI: https://doi.org/10.1094/PDIS-09-20-1873-RE

Interpretive Summary: Rhododendrons are an important component of the ornamental nursery industry, but are prone to Phytophthora root rot. Phytophthora root rot is a continuing issue on rhododendrons despite decades of research. Several Phytophthora species are known to cause root rot, but most research has focused on P. cinnamomi and information on pathogenicity is limited or missing for other commonly encountered oomycetes, including P. plurivora, P. pini, P. pseudocryptogea, and P. cambivora. In this study, 3 to 4 individuals of each pathogen were used to inoculate rhododendron plants. Phytophthora cinnamomi, P. plurivora, and P. pini frequently caused severe disease, P. pseudocryptogea caused moderate to severe disease, and P. cambivora rarely caused severe disease. No differences in pathogenicity were observed among individuals of same species, except for one isolate of P. plurivora, which caused less disease than the other individuals of this species. Results show that in addition to P. cinnamomi and P. plurivora, that P. pini and P. pseudocryptogea are important pathogens also capable of causing severe root rot in rhododendron. Further research is needed to evaluate whether differences among pathogen species affect disease control.

Technical Abstract: Phytophthora root rot is a destructive disease of rhododendron, causing substantial losses of this important ornamental nursery crop in infested fields 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 now more common than P. cinnamomi and that several other Phytophthora species may also be involved. We investigated the ability of the five most abundant species identified from the survey to cause root rot: P. cambivora, P. pseudocryptogea, P. cinnamomi, P. pini, and P. plurivora. We infested the soil of containerized Rhododendron catawbiense ‘Boursault’ in a greenhouse experiment using three to four isolates of each species obtained from six Oregon nurseries. Four species caused moderate to severe disease, with P. plurivora, P. cinnamomi, and P. pini rapidly causing = 90% incidence of severe root rot while P. pseudocryptogea caused more moderate disease with 46% incidence of severe root rot. Phytophthora cambivora inoculum failed to reach the same inoculum density as the other four pathogen species, but occasionally caused severe root rot (5% incidence). No differences in pathogenicity were observed among isolates of same species, except for one isolate of P. plurivora. This study demonstrates that all five Phytophthora species tested, which were representative of 94% of the isolates from the survey, are capable of causing severe root rot and plant death, but that not all species are equally pathogenic. Further research on Phytophthora root rot of rhododendron should consider whether the efficacy of disease control strategies, such as host resistance and chemical control, differ among the causal agents of this disease.