2025/09/02 by Chaydon O’Fallon, Wei Hao Belisle, Greg T. Browne +2 · 1 voice · 1 citation
Agricultural and Biological Sciences · Biochemistry, Genetics and Molecular Biology · #Plant Pathogens and Resistance #Plant Pathogens and Fungal Diseases #Plant Disease Resistance and Genetics
paper · doi:10.1094/pdis-05-25-1038-re
openalex publication_date 2025/09/02 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/18
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 effective concentration of fungicide required for a 50% effective concentration (EC 50 ) 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 EC 50 values ≤ 0.289 μg/ml. Phytophthora niederhauserii and P. syringae were also sensitive to potassium phosphite (KPO 3 ; EC 50 values 2.8 μg/ml to 60.8 μg/ml); however, all but 4 of the 63 isolates of P. mediterranea were resistant (EC 50 values between 50.9 μg/ml and 464.3 μg/ml). Fungicidal versus 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 KPO 3 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 for almond, and registration for the other three fungicides is planned.