Bacillus velezensis synergizes with reduced tebuconazole doses to control DMI-resistant Alternaria alternata by suppressing AaCYP51 expression

Demethylation inhibitor fungicides (DMIs) such as tebuconazole have been widely used to control Alternaria alternata , but their intensive application has led to the emergence of resistant populations, primarily associated with overexpression of the target gene AaCYP51 . In this study, we investigated whether metabolites of Bacillus velezensis could suppress AaCYP51 expression and enhance the efficacy of tebuconazole against resistant A. alternata isolates. The combined treatment with B . velezensis 1 % (v/v) and 5 µg/mL tebuconazole, significantly reduced expression levels of AaCYP51 in both tebuconazole-resistant and sensitive isolates. Detached cherry fruit assay revealed that solo application of B . velezensis or tebuconazole (86 µg/mL) showed very limited control efficacy, but their combined application resulted in significantly higher control efficacy for resistant isolates. In contrast, all treatments fully suppressed disease caused by sensitive isolates. These results indicate that B . velezensis increases the efficiency of tebuconazole against A . alternata by reducing the expression of AaCYP51 , which provides a mechanistic basis for combined use of biological control agents and fungicides, resulting in reduced fungicide input and sustainable disease management in cherry orchards.