Increasing virulence and decreasing fungicide sensitivity in Phytophthora capsici after continuous metalaxyl-chlorothalonil exposure

Several fungicide products with metalaxyl-chlorothalonil as the active ingredients are available in the Mexican market. Applying products containing this mixture of molecules is a common method that farmers choose to prevent damage caused to crops by Phytophthora capsici. Despite the efficacy of the mixture of metalaxyl-chlorothalonil in controlling the growth of this pathogen, their intensive use could contribute to the development of multidrug-resistant strains. Additionally, the dynamics of resistance to commercial metalaxyl-chlorothalonil-based fungicides and the virulence of P. capsici during continuous exposure are yet to be studied in detail. In this study, four continuous generations of P. capsici were exposed to two concentrations (5 and 100 μg·mL⁻¹) of a commercial fungicide containing metalaxyl-chlorothalonil as the active ingredients. Changes in morphology, growth, and virulence were observed after the first interaction with the fungicide. However, in further generations exposed to the same amount of the product, the morphology and growth of P. capsici were partially restored, whereas its virulence was completely re-established. These results indicate a rapid adaptation to the metalaxyl-chlorothalonil mixture and highlight the importance of using different methods to control P. capsici and prevent the development of more virulent and multidrug-resistant strains.