Pre-plant soil fumigation of new orchards to reduce pathogen populations is highly regulated, and older fumigants are being phased out due to safety and climate concerns. Pathogens such as Phytophthora spp. can cause significant losses, especially to young orchards. Older registered Oomycota fungicides in California are phosphonates (FRAC Code - FC P07/33) and phenylamides (FC 4). New fungicides registered or planned for registration include oxathiapiprolin (FC 49), mandipropamid (FC 40), fluopicolide (FC 43), and ethaboxam (FC 22). These fungicides are applied post-planting and it is unknown if they permanently inactivate or only temporarily inhibit pathogen propagules. We developed an assay to evaluate these chemicals for their fungicidal and fungistatic activities against encystedzoospores. Cysts of P. citrophthora were exposed to these fungicides at standardized concentrations based on field rates for two hours, washed, and the incidence of germination or presence of formazan after a colorimetric vital stain with MTT were determined. All fungicides significantly reduced cyst germination and live cell count when compared to the control. Fluopicolide and oxathiapiprolin had highest fungicidal activity, whereas phosphite was mostly fungistatic. These results indicate that the fungicides evaluated have differential effects at the utilized dose and exposure time. Some have the potential to reduce populations of Phytophthora spp. in soils as post-plant treatments.
