Soil Solarization to Eradicate Soilborne Phytophthora spp. in Container Nurseries with Surface Gravel1

To describe the effect of soil solarization in the presence of a gravel layer on the soil surface of container nurseries, we investigated belowground temperatures and soil water potential during solarization with different thicknesses of a surface gravel layer (2.5 cm, 7.5 cm, or no gravel) (1 in, 3 in, or no gravel) in relation to survival of soilborne Phytophthora spp. inoculum. In field trials conducted for 4 weeks with Phytophthora ramorum Werres and Phytophthora pini Leonian in San Rafael, California and with P. pini in Corvallis, Oregon, infested rhododendron leaf inoculum was placed on the surface, and at 5 cm (2 in) and 15 cm (6 in) below the surface. In solarized plots with thicker layers of gravel, inoculum buried in the soil layer was killed in shorter treatment periods by higher elevated temperatures. Inoculum at the surface and within the gravel layer was also killed, but showed greater tolerance to heat under the lower water potential conditions as compared to the soil layer. P. pini has a significantly longer survival in heat than P. ramorum, allowing it to serve as a conservative surrogate for P. ramorum in testing solarization outside the quarantine facility. This study demonstrates how presence of a gravel layer influences soil solarization effectiveness in reducing Phytophthora inoculum survival. Index words: Phytophthora ramorum, Phytophthora pini, soil disinfestation, disease management, soil temperature, soil water potential, ornamentals. Species used in this study: Phytophthora ramorum Werres, de Cock & Man in't Veld, Phytophthora pini Leonian.