Discovery of Frenolicin B as Potential Agrochemical Fungicide for Controlling Fusarium Head Blight on Wheat

In this study, the supernatant extract from fermentation broth of Streptomyces sp. NEAU-H3 showed strong antifungal activity against Fusarium graminearum strain PH-1 in vitro and in vivo. Three known pyranonaphthoquinones were isolated by means of an activity-guided method, and frenolicin B was characterized as the main active ingredient. Frenolicin B displayed strong antifungal activity against F. graminearum strain PH-1 with an EC50 value of 0.51 mg/L, which is lower than that of carbendazim (0.78 mg/L) but higher than that of phenamacril (0.18 mg/L). Frenolicin B could also strongly inhibit the mycelial growth of Fusarium species, including F. graminearum and F. asiaticum, as well as carbendazim-resistant Fusarium strains isolated from field, with EC50 values of 0.25–0.92 mg/L. Results from field experiments showed that the efficacy of frenolicin B in controlling Fusarium head blight at a treatment concentration of 75 g ai/ha was better than those of phenamacril (375 g ai/ha) and carbendazim (600 g ai/ha) or had no significant difference with that of phenamacril (375 g ai/ha) in 2 years. Scanning electron microscope and transmission electron microscope observations revealed that after treating F. graminearum mycelia with frenolicin B, the mycelia appeared aberrant and had an uneven thickness and swelling, the cytoplasm had disintegrated, and some cell contents were lost. Transcriptome analysis suggests that frenolicin B might inhibit the metabolism of nucleotides and energy by affecting genes involved in phosphorus utilization but did not affect the expression of myosin 5, which is the specific target of phenamacril. These findings indicate that frenolicin B may be a potential agrochemical fungicide for controlling Fusarium head blight.