Sensitivity of the causal agent of Phytophthora blight of peppers, Phytophthora capsici, to mefenoxam and new oomycota fungicides

Phytophthora capsici is a destructive oomycete pathogen causing rot and blight in peppers and other vegetable crops, leading to significant yield losses worldwide. Multiple  fungicides with different modes of actions are used to manage this pathogen. A population of 19 P. capsici isolates, collected from a chile pepper farm in Arizona in 2024,  were evaluated  for sensitivity to eight fungicides, which included dimethomorph, ametoctradin + dimethomorph, fluopicolide, ethaboxam, mandipropamid, oxathiapiprolin, cyazofamid, and mefenoxam.  Fungicide sensitivity of P. capsici was assessed using fungicide-amended media to determine the effective concentration needed to inhibit mycelial growth by 50%. The growth of 57% (4/7) of isolates was completely inhibited by 0.0005 µg/mL of oxathiapiprolin. In 42% (8/19) of the isolates, mycelial growth was  significanty reduced by 0.25 µg/mL of ethaboxam. However, mycelial growth in the remaining isolates was only partially inhibited at these concentrations of the fungicides. Understanding the in vitro and in vivo sensitivity of P. capsici populations to commonly used fungicides is critical for developing sustainable disease management strategies and preventing fungicide resistance development. Continuous fungicide sensitivity screening is crucial for monitoring the development of fungicide resistance in populations of P. capsici in the U.S.