Survival and dispersal of Phytophthora multivora zoospores in soil substrates

Phytophthora pathogens are responsible for causing disease in a range of environments, including natural, urban, nurseries and horticultural settings, and can be cryptically spread when present as inoculum in infected soil media. By evaluating the survival of Phytophthora inoculum in soil substrates without plants, their potential to be spread cryptically could be better understood. We tested the ability of Phytophthora multivora to survive in and be dispersed from sterile potting mix, forest soil and sand in the absence of plants when introduced as zoospores. We found that P. multivora zoospores readily encysted and survived in the potting mix for up to 76 days. The encysted zoospores were dispersed from the inoculated potting mix pots by overhead watering for up to 49 days. Zoospore cysts accumulated in the bottom sections of the potting mix pots as they were washed downward with each watering event. In contrast, P. multivora did not survive and persist in the sterile sand and forest soil substrates well. At 28 days-post-inoculation, only 10.1% of the destructively harvested replicates were positive with baiting for both sand and soil, while 100% of the potting mix reps were positive. The results raise concerns about the cryptic dispersal of inoculum during restoration projects especially from potting mix and show that zoospore cysts can contribute to the longer-term survival of Phytophthora inoculum. The potential presence of cryptic Phytophthora inoculum in nursery plants should be considered when sourcing plants for restoration projects to avoid inadvertently spreading soil-borne Phytophthora diseases.