Field Evaluation of a Novel, Granular Soil Fumigant for Controlling Phytophthora ramorum in Field Nursery Soils

Phytophthora ramorum, the causal agent of Sudden Oak Death (SOD) and ramorum blight, infects a wide range of hardwood and nursery ornamental species. Chlamydospores of P. ramorum can survive for extended periods of time in soils. Two studies were conducted, including: 1) a laboratory study to evaluate two liquid disinfectants for controlling P. ramorum chlamydospores, and 2) a field study to evaluate a novel soil fumigation treatment as an alternative to soil steaming or methyl bromide soil fumigation. The liquid disinfectants were ElectroBiocide and Oxidate 2.0. The laboratory study resulted in complete inactivation of the P. ramorum chlamydospores after six minutes of contact time for both the liquid disinfectants The field study evaluated a chlorine dioxide granule formulation that was applied at two rates in a nursery soil. Rhododendron leaf discs were inoculated with P. ramorum, placed in permeable sachets and buried at two soil depths in a research nursery. Soil treatments also included saturated hydrogels (with and without gels) so that soil moisture effects on chlamydospore survival could be estimated. The sachets were recovered 5, 15 and 30 days after the soil treatment. Efficacy of the soil treatments was evaluated by the number of leaf discs showing P. ramorum growth recovered from the sachets. The soil fumigation treatment with highest efficacy occurred when the sachets were buried at the 5 cm soil depth, were treated with hydrogels, at the highest Z-series granule rate (800g/tube), and had a contact time of 30 days. The probability of P. ramorum growth for this soil treatment was 0.18, or 18%, i.e. the probability of that fumigation treatment inactivating the pathogen was 82%. Also, as the soil moisture increased, the efficacy of the fumigation treatments also increased.