Effects of Soil Solarization and Trichoderma asperellum on Soilborne Inoculum of Phytophthora ramorum and Phytophthora pini in Container Nurseries

Infested container nursery beds are an important source of soilborne Phytophthora spp. for initiating disease through movement with surface water or splashing onto foliage. We investigated the effects of soil solarization, alone or with subsequent amendment with a Trichoderma asperellum biocontrol agent, on the survival of Phytophthora spp. inoculum. In field trials conducted with Phytophthora ramorum in San Rafael, CA and with P. pini in Corvallis, OR, infested rhododendron leaf inoculum was buried at 5, 15, and 30 cm below the soil surface. Solarization for 2 or 4 weeks during summer 2012 eliminated recovery of Phytophthora spp. buried at all depths in California trial 1, at 5 and 15 cm in California trial 2, but only at 5 cm in Oregon. There was no significant reduction of Phytophthora spp. recovery after T. asperellum application. Although the population densities of the introduced T. asperellum at the 5-cm depth were often two- to fourfold higher in solarized compared with nonsolarized plots, they were not significantly different (P = 0.052). Soil solarization appears to be a promising technique for disinfesting the upper layer of soil in container nurseries under certain conditions.