Inhibitory activity of three types of fungicides on Cladosporium paeoniae and their control efficacy against paeony leaf mold

Leaf mold, which is cause by Cladosporium paeoniae, is one of the most serious diseases on peony, causing reduced growth, flower discolouration and peony seed oil reduction. In order to screen fungicides for the control of peony leaf mold, this paper investigated the inhibitory activity of four fungicides, pyraclostrobin, phenamacril, flutolanil and boscalid, on the growth and development of Cladosporium paeoniae using the coating plate method, and further determined the field control efficacy of the fungicides against leaf mold using fungicide sprays. The results showed that pyraclostrobin had strong inhibitory activity against all developmental stages of the fungus, with EC₅₀ of 0.0045, 0.0013, 0.0184 and 0.0072 μg/mL for conidial germination, germ tube elongation, mycelial growth and conidial production, respectively. Phenamacril strongly inhibited germ tube elongation and conidial production, with EC₅₀ of 0.3358 and 0.1368 μg/mL respectively, and also inhibited conidial germination, but had little effect on mycelial growth. Both flutolanil and boscalid showed weak inhibition of conidial germination and germ tube growth with EC₅₀ above 25 μg/mL. Boscalid had strong inhibitory effects on mycelial growth and conidia production, with EC₅₀ less than 2.2 μg/mL, but flutolanil had almost no inhibitory effects on mycelial growth and could even promote sporulation of the pathogen. Although all four fungicides inhibited elongation of germ tubes, none of them had significant teratogenic effects on germ tubes and conidia production. Compared with the control group without any spraying, all four fungicides significantly decreased the disease index in the field after application, and all achieved more than 60% control efficacy on the disease; with the increase of the dilution of the fungicide, the control efficacy was reduced. Pyraclostrobin and phenamacril are recommended for use as protective fungicides before the appearance of leaf mold, while after disease occurrence, pyraclostrobin and boscalid are recommended.