Soilborne Phytophthora and Pythium diversity from rhododendron in propagation, container, and field production systems of the Pacific Northwest

Authors: Weiland, Gerald - Jerry; Scagel, Carolyn; Grunwald, Niklaus - Nik; Davis, Elizabeth; Beck, Bryan; FOSTER, ZACHARY - Oregon State University; FIELAND, VALERIE - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2020
Publication Date: 5/5/2020
Citation: Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R., Foster, Z.S., Fieland, V.J. 2020. Soilborne Phytophthora and Pythium diversity from rhododendron in propagation, container, and field production systems of the Pacific Northwest. Plant Disease. 104(6):1841-1850. https://doi.org/10.1094/PDIS-08-19-1672-RE.
DOI: https://doi.org/10.1094/PDIS-08-19-1672-RE

Interpretive Summary: Rhododendrons, grown for their spring flowers, are an important part of the nursery industry valued at $42 million. Unfortunately, rhododendrons are susceptible to root rot, which can be caused by many different pathogens. Little is known about which pathogens cause root rot on this crop since the last pathogen survey was conducted over 40 years ago. Since then, other pathogens may have been introduced into the industry, and is important to know the identify of the pathogens causing root rot in order to more effectively treat the disease. We surveyed seven rhododendron nurseries and identified 31 pathogens associated with root rot. Root rot damage was rare during the propagation stage. However, severe root rot was common in container- and field-grown plants where Phytophthora pathogens were also more common. One grower lost 90% of their rhododendrons valued at $25,000 in one field alone. This tells us that Phytophthora species are the main cause of root rot and that most contamination by these pathogens comes in after the plants have been propagated. We also discovered that one Phytophthora species, P. plurivora, has become more widespread than in the past. This information is important because most disease control options for rhododendron root rot are targeted towards a different Phytophthora species, P. cinnamomi. These options may not be as effective for P. plurivora and may help explain why root rot is still a significant problem for the rhododendron industry. Researchers are currently evaluating how well existing disease control methods work for different root rot pathogens.

Technical Abstract: Rhododendron root rot is a severe disease that kills rhododendrons. Information is needed about the soilborne Phytophthora and Pythium species causing root rot in order to better understand the disease and to optimize disease control The last survey focusing solely on oomycete soilborne pathogens in rhododendron production was conducted in 1974. Since then, advances in pathogen identification have occurred, new species may have been introduced, pathogen communities may have shifted, and little is known about Pythium species affecting this crop. Therefore, a survey of root-infecting Phytophthora and Pythium species was conducted at seven nurseries to: 1) document the amount of root rot at each nursery and production stage; 2) identify soilborne species infecting rhododendron; and, 3) determine if there are differences in pathogen diversity among production systems and nurseries. Rhododendrons from propagation, container, and field systems were sampled and Phytophthora and Pythium species were isolated from the roots and collar region. Root rot was rare in propagation systems, which were dominated by Pythium. However, severe root rot was common in container and field systems where Phytophthora was more prevalent, suggesting that Phytophthora species are the primary cause of severe root rot and that most contamination by these pathogens comes in after the propagation stage. Pythium cryptoirregulare, Py. aff. macrosporum, P. plurivora, and P. cinnamomi were the most frequently isolated species at most nurseries and results showed that P. plurivora has become much more widespread than in the past. Phytophthora diversity was also greater in field systems than in propagation or container systems. Risks for Phytophthora contamination were identified and included contaminated potting media or soil, dead plants as sources of inoculum, and excess water. In the past, research on disease development and root rot control in rhododendron has focused almost exclusively on P. cinnamomi. More research is needed on the other root-infecting species identified in this survey.