In vitro evaluation of antifungal activity of some agricultural fungicides against two saprolegnoid fungi infecting cultured fish

Background: Saprolegniasis often cause a significant economic losses to fish hatchery and intensive fish industry. The treatment of saprolegnoid diseases with malachite green seems to have harmful effect and is considered as a mutagenic and carcinogenic substance. The teratogenic and carcinogenic potential of malachite green increased the necessity to find an effective alternative treatment to control the disease. Methods: In the present study, seven fungicides used in agriculture were investigated to control Saprolegnia parasitica and S. diclina in vitro. The antifungal activity of each fungicide was compared with that of malachite green as reference fungicide using food poisoning technique. Fish toxicity of the effective fungicides was performed to detect the most applicable fungicides in fish aquarium. Results: Four of seven fungicides were found to be effective against the two saprolegnoid fungi. Azoxystrobin and metalaxyl were the most effective fungicides inhibited fungal growth of the two saprolegnoid fungi completely at concentration of 200 ppm and 250 ppm respectively while cyazofamid and etridiazole were moderately effective. Acute toxicity assay of the two effective fungicides showed metalaxyl was low toxic to fish with LC50 of 360 ppm while azoxystrobin was 225 ppm. Hence, it was confirmed that 200 and 250 ppm of azoxystrobin and metalaxyl which completely inhibited mycelial growth of the saprolegniod fungi could be safely used for saprolegniasis control. Conclusion: These fungicides which proved to be effective and fishery safer may be applicable as a aquatic fungicides avoiding teratogenic and carcinogenic risk of malachite green. (C) 2020 The Author(s). Published by Elsevier B.V. on behalf of King Saud University.