Soil Temperature and Moisture Conditions Affect the Recovery and Sporulation Capacity of <i>Phytophthora ramorum</i> from Infested <i>Rhododendron</i> Leaf Disks

The invasive pathogen <i>Phytophthora ramorum</i> 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 the inoculum at 4°C increases the number of sporangia produced from infested rhododendron leaf disks. In a second experiment, the inoculum was incubated in soil at a range of temperature (6.7, 14, 20, or 28°C) and soil moisture (approximately 0, -40, or -400 kPa) conditions for up to 18 weeks. Our ability to culture <i>P. ramorum</i> 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 the inoculum incubated at both 20 and 28°C in all soil moisture conditions. However, subsequent incubation of this inoculum at 4°C for an additional 7 weeks restored sporulation potential, at times exceeding pre-incubation levels. These results are consistent with field observations that <i>P. ramorum</i> becomes more biologically active after exposure to cooler temperatures and highlights the risk a 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.