Toxicity of registered and new fungicides to current species of Phytophthora infecting almond in California and their fungicidal properties

Phytophthora diseases have become increasingly important in California almond production and often seriously impact orchard establishment. Phytophthora mediterranea and P. niederhauserii isolated from crown and trunk cankers, and P. syringae mostly obtained from aerial Phytophthora infections were common species recovered. This represents a shift in species occurring on almond. Mycelial growth of a total of 195 isolates of these species was effectively inhibited by ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin with EC50 values of 0.006 to 0.190 µg/ml, 0.006 to 0.317 µg/ml, 0.001 to 0.011 µg/ml, and 0.00011 to 0.00041 µg/ml, respectively, with similar ranges for each species. All isolates were sensitive to mefenoxam with EC50 values ≤0.289 µg/ml. P. niederhauserii and P. syringae were also sensitive to potassium phosphite (KPO3; EC50 values 2.8 µg/ml to 60.8 µg/ml), however, all but 4 of the 63 isolates of P. mediterranea were resistant (EC50 values between 50.9 µg/ml and 464.3 µg/ml). Fungicidal vs. fungistatic activity of the six fungicides at concentrations proportionally scaled based on field rates was determined by vital staining and germination of zoospore cysts of P. citrophthora that were fungicide-treated and then washed. Each fungicide significantly reduced cyst viability. Fluopicolide and oxathiapiprolin had the highest fungicidal activity, whereas KPO3 was mostly fungistatic. With high in vitro toxicity and high direct fungicidal activity at low concentrations, the new ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin have the potential to effectively manage Phytophthora diseases and also reduce pathogen populations. Oxathiapiprolin has been registered on almond, and registration for the other three fungicides is planned.