Antifungal Activity of the Novel Fungicide Cyazofamid against Phytophthora infestans and Other Plant Pathogenic Fungi in Vitro

Antifungal activity of the novel fungicide cyazofamid (ISO proposed common name), 4-chloro-2-cyano-N,N-dimethyl-5-ptolylimidazole-1-sulfonamide, was investigated. Cyazofamid proved highly selective to fungi classified as Oomycetes in mycelial growth inhibition tests. The EC50 values of cyazofamid against Phytophthora infestans, Phytophthora sojae, Pythium aphanidermatum, Pythium paddicum, Pythium sylvaticum, and Aphanomyces cochlioides were between 0.008 and 0.2 MU g/ml. On the other hand, cyazofamid did not inhibit mycelial growth of fungi classified as Ascomycetes, Basidiomycetes, or Deuteromycetes. Cyazofamid strongly inhibited all stages in the life cycle of P. infestans. Minimum inhibitory concentrations (over 90% inhibition) against indirect germination of zoosporangia (zoospore release), zoospore motility, cystospore germination, and oospore formation were 0.1–0.5, 0.005, 0.05, and 0.01 MU g/ml, respectively. Cyazofamid at 0.1 MU g/ml exhibited complete fungicidal activity on zoospore release by P. infestans 60 min after treatment. When 20 MU M ATP was applied to a zoosporangia suspension before addition of 0.05–0.1 MUg/ml cyazofamid, inhibition of zoospore release was reduced. These results suggest that the mode of action of cyazofamid is connected to impairment of the ATP energy generation system.