Soil temperature and moisture conditions affect the recovery and sporulation capacity of Phytophthora ramorum from infested Rhododendron leaf disks

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

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2025
Publication Date: 8/7/2025
Citation: Peterson, E.K., Grünwald, N.J., Parke, J.L. 2025. Soil temperature and moisture conditions affect the recovery and sporulation capacity of Phytophthora ramorum from infested Rhododendron leaf disks. Journal of Phytopathology. 115(8):968-977. https://doi.org/10.1094/PHYTO-02-25-0056-R.
DOI: https://doi.org/10.1094/PHYTO-02-25-0056-R

Interpretive Summary: Sudden oak death is caused by the plant pathogen Phytophthora ramorum. This pathogen is a water mold and known to persist in soil and require wet, water-logged conditions for infecting host plants. We evaluated how the pathogen survives in soil depending on different conditions of soil temperature and soil moisture. We found that plant infection is promoted by exposure to low temperatures around 4°C. This work provides novel insights on the survival and the ability to infect plants of P. ramorum in soils.

Technical Abstract: The invasive pathogen Phytophthora ramorum persists within nurseries, potentially within buried, infested leaf debris. To determine how the environment – notably soil temperature and moisture levels – affects the epidemiological risk of soil inoculum reserves, we performed laboratory assays assessing how variable conditions impact pathogen survival and its capacity to sporulate. We first established that incubating inoculum at 4°C increases the number of sporangia produced from infested rhododendron leaf disks. In a second experiment, inoculum was incubated in soil at a range of temperatures (6.7, 14, 20, or 28°C) and soil moisture (0, -40, or -400 kPa) conditions for up to 18 weeks. Our ability to culture P. ramorum was only negatively affected by the warmest and driest regimes. In contrast, the capacity to sporulate was affected over a much wider range of conditions, whereby declines in sporulation potential were observed over time from inoculum incubated at both 20 and 28°C (all soil moisture conditions). However, subsequent incubation of this inoculum at 4°C for an additional seven weeks restored sporulation potential, at times exceeding pre-incubation levels. These results are consistent with field-observations that P. ramorum becomes more biologically active after exposure to cooler temperatures, and highlights the risk soilborne inoculum poses during some times of the year. Disinfestation of soils through artificially high heat is likely required to prevent recurrent infections within nurseries from soilborne sources, and thus prevent the further spread of this invasive pathogen.