Professor of Plant Pathology - Fruit Crops Clemson University Clemson, South Carolina, United States
Brown rot, caused by Monilinia fructicola, is a major threat to peach production in the Southeastern U.S. Although DMI fungicides are standard tools, their efficacy is reduced due to overexpression of the MfCYP51 target gene. This study evaluated the compatibility and synergism of low-dose DMI fungicides with the biorational products Theia (Bacillus subtilis) and Howler EVO (Pseudomonas chlororaphis). In vitro tests showed that difenoconazole, metconazole, and tebuconazole inhibited B. subtilis growth, while flutriafol and mefentrifluconazole did not. Detached fruit assays and a one-year field trial on nectarines revealed synergistic effects between Howler EVO and sub-label rates of propiconazole and control efficacy, comparable to the grower standard. No synergy was observed for Theia with mefentrifluconazole. Mycelia from three DMI-sensitive and three DMI-resistant M. fructicola isolates were treated with propiconazole (0.3 µg/ml), Howler EVO (88.1 µg/ml), Theia (209.5 µg/ml), or their combinations for 6 hours prior to RNA extraction. Gene expression analysis showed that Howler EVO suppressed both constitutive and DMI-induced MfCYP51 expression, while Theia had no effect or increased expression in resistant isolates. These findings highlight the potential of compatible biorational–DMI mixtures, particularly those involving Howler EVO, to improve disease control and reduce synthetic fungicide use.
