Risk of Epidemic Development in Nurseries from Soil Inoculum of .

Soilborne inoculum arising from buried, infested leaf debris may contribute to the persistence of at recurrently positive nurseries. To initiate new epidemics, inoculum must not only survive but also 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 noninfested leaf disks (2014 only) and then retrieved to assess pathogen viability and sporulation capacity every 5 weeks. Two 14-week-long trials were conducted in 2016. We were able to consistently culture 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. 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 from inoculum buried at 15 cm the previous June. We validate prior observations that poses a greater risk after exposure to cooler temperatures and provide evidence that infested leaf debris plays a role in the perpetuation of in nurseries. This work provides novel insights into the survival and epidemic behavior of in nursery soils.