In Vitro Screening of Fungicidal Chemicals for Antifungal Activity against Saprolegnia

Saprolegnia is an important fish fungal pathogen that often results in significant economic losses to freshwater aquaculture. To find effective drugs to control saprolegniasis, 30 fungicidal chemicals used in agriculture were screened, in which kresoxim‐methyl and azoxystrobin, with minimum inhibitory concentration (MIC) values of 1.0 and 0.5 mg/L, respectively, showed good in vitro antifungal activities against Saprolegnia. Azoxystrobin has the most promising anti‐Saprolegnia activity with 50% effective concentration (EC₅₀) value of 0.212 mg/L against mycelial growth and minimum fungicidal concentration (MFC) value of 0.13 mg/L against spores, while EC₅₀ and MFC values to kresoxim‐methyl are 0.240 and 0.25 mg/L, respectively. Through the acute toxicity assay using goldfish, Carassius auratus, azoxystrobin exhibited wider margin of safety with a safe concentration (SC) value of 0.553 mg/L than kresoxim‐methyl with an SC value of 0.131 mg/L. These findings demonstrated that azoxystrobin has the potential for the development of therapy for the control of Saprolegnia in aquaculture. Both kresoxim‐methyl and azoxystrobin were tested with a post‐antifungal effects (PAFE) assay and the results revealed that the two chemicals had no significant effect on fungal growth inhibition after a 1‐hour exposure, indicating that the treatment needs to be carried out over an extended period.