Biological activity of seven fungicides on Cladosporium paeoniae and their field control efficacy against red spot of oil peony

Red spot of oil peony, caused by Cladosporium paeoniae, severely affects the ornamental value of oil peony and threatens the oil yield of its seeds, significantly reducing its economic value. Currently, only azoxystrobin has been registered for the control of this disease, resulting in limited fungicide options. This study evaluated the inhibitory activities of seven fungicides—pydiflumetofen, fluopyram, prochloraz, fluazinam, iprodione, prothioconazole, and tebuconazole—against four key developmental stages of C. paeoniae (spore germination, germ tube elongation, mycelial growth, and sporulation) via in vitro toxicity assays, using azoxystrobin as a control. In addition, field efficacy trials were conducted during the growing season of oil peony using spray applications. The results showed that all seven fungicides inhibited different developmental stages of C. paeoniae, but with distinct activity spectra. Prochloraz and pydiflumetofen demonstrated strong, broad-spectrum inhibition across all four stages, with EC 50 values of 0.0083-0.0756 μg/mL and 0.0047-0.6990 μg/mL, respectively. Fluazinam and tebuconazole were particularly effective against germ tube elongation (EC 50 = 0.0106 μg/mL) and mycelial growth (EC 50 = 0.3166 μg/mL), respectively. Fluopyram and prothioconazole showed moderate effects on germ tube elongation and sporulation but weak activity on spore germination and mycelial growth. Iprodione was the least effective overall. Field trials further demonstrated that all fungicide treatments significantly reduced the disease index of red spot, with prochloraz achieving the best control (> 85%). These results indicate that prochloraz and pydiflumetofen possess strong and broad-spectrum antifungal potential against C. paeoniae, providing new options for the chemical control of red spot of oil peony.