Risk of epidemic development in nurseries from soil inoculum of Phytophthora ramorum

Authors: PETERSON, EBBA - Oregon State University; Grunwald, Niklaus - Nik; PARKE, JENNIFER - Oregon State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2021
Publication Date: 3/31/2022
Citation: Peterson, E.K., Grunwald, N.J., Parke, J.L. 2022. Risk of epidemic development in nurseries from soil inoculum of Phytophthora ramorum. Phytopathology. 112(5):1046-1054. https://doi.org/10.1094/PHYTO-06-21-0245-R.
DOI: https://doi.org/10.1094/PHYTO-06-21-0245-R

Interpretive Summary: Sudden oak death, caused by the plant pathogen Phytophthora ramorum, is causing epidemics in oaks forests on the US West Coast and twig and foliar diseases in numerous native and non-native ornamental plants, shrubs, and trees. It is not clear how the pathogen survives as spores in nursery environments. We evaluated how the pathogen survives in nurseries in controlled field experiments. Spores present in soil gradually lose the ability to infect plants; however, spore production varied seasonally and was higher during cooler temperatures. This work provides novel insights into the survival and disease development of the sudden oak death pathogen in nursery soils. This work will be used to determine the risk of infection in nurseries by regulatory agencies monitoring the disease in commercial nurseries.

Technical Abstract: Soilborne inoculum arising from buried, infested leaf debris may contribute to the persistence of Phytophthora ramorum at recurrently positive nurseries. To initiate new epidemics, inoculum must not only survive, but produce sporangia during times conducive to infection at the soil surface. To assess this risk, we performed two year-long experiments in a soil plot at the National Ornamentals Research Site at Dominican University of California. Inoculated rhododendron leaf disks were buried at a depth of 5 or 15 cm in the early summer of 2014 or 2015. Inoculum was baited at the soil surface with non-infested leaf disks (2014 only), then retrieved to assess pathogen viability and sporulation capacity every five weeks. Two 14-week-long trials were conducted in 2016. We were able to consistently culture P. ramorum over all time periods. Soil incubation rapidly reduced the capacity of inoculum to sporulate, especially at 5 cm; however, sporulation capacity increased with the onset of seasonally cooler temperatures. P. ramorum was baited most frequently between November and January, especially from inoculum buried at 5 cm 1-day before the baiting period; in January we also baited P. ramorum from inoculum buried at 15 cm the previous June. We validate prior observations that P. ramorum poses a greater risk after exposure to cooler temperatures, and provide evidence that infested leaf debris plays a role in the perpetuation of P. ramorum in nurseries. This work provides novel insights into the survival and epidemic behavior of P. ramorum in nursery soils.