Abstract: 【Objective】The objective of this study is to make clear the activity of fludioxonil on mycelial growth, conidia germination, germ tube elongation and sporulation of four peony foliar pathogenic fungi, including Alternaria suffruticosae, Phyllosticta commonsii, Hainesia lythri and Cladosporium paeoniae, and to analyze whether fludioxonil could be used in the chemical control of peony disease. 【Method】The inhibitory activity of fludioxonil against mycelial growth was measured through a mycelial linear growth test and the efficacy of fludioxonil on conidial germination, germ tube elongation and sporulation was determined on the PSA or KDA amended with fludioxonil at various concentrations. 【Result】Fludioxonil had a strongest inhibition on mycelial growth of H. lythri with the EC₅₀ of 0.01 μg·mL^-1, followed by 0.07 μg·mL^-1 of A. suffruticosae and 0.35 μg·mL^-1 of P. commonsii. Fludioxonil could strongly inhibit the conidia germination of four peony pathogenic fungi and the inhibitory activity on H. lythri was the strongest, with the EC₅₀ of1.26 μg·mL^-1. The EC₅₀ to other three fungi ranged from 3.27 to 3.45 μg·mL^-1. Fludioxonil could also strongly inhibit the germ tube elongation of four fungi, and the suppression ratio of 0.1 μg·mL^-1 fludioxonil reached 40% to 70%. The germ tube of H. lythri was the most sensitive to fludioxonil, and the EC₅₀ was 0.04 μg·mL^-1. The EC₅₀ of fludioxonil to other three fungi’s germ tube elongation varied from 0.08 to 0.22 μg·mL^-1. Fludioxonil could cause the abnormality of conidia and the germ tube of A. suffruticosae and P. commonsii, including expanding of spore and germ tube and excessively branching of germ tube. However, the effect of fludioxonil on that of H. lythri and C. paeoniae was unconspicuous. Fludioxonil could delay sporulation of A. suffruticosae, P. commonsii and C. paeoniae. The EC₅₀ of A. suffruticosae sporulation was 0.05 μg·mL^-1, followed by the 0.38 μg·mL^-1 of C. paeoniae, whereas fludioxonil could promote the sporulation of H. lythri.【Conclusion】Fludioxonil had a strong inhibitory activity on the mycelial growth, conidia germination and germ tube elongation of A. suffruticosae and H. lythri, but could promote the sporulation of H. lythri. Fludioxonil could also inhibit powerfully conidia germination and germ tube elongation of P. commonsii and C. paeoniae, mycelial growth of P. commonsii and sporulation of C. paeoniae.Fludioxonil should be used preferentially as a curative fungicide to avoid the pathogenic fungi infection, as it could not be absorbed by the plant tissue and inhibit the fungi that had penetrated and parasitized the plant.

Key words: fludioxonil, peony pathogenic fungi, mycelial growth, conidia germination, sporulation, germ tube elongation