Influence of propiconazole and metconazole formulations on Bacillus subtilis vegetative cell growth and disease control of fruit crops.

Biological control agent (BCA) Bacillus subtilis formulated as Theia® is registered for control of fungal and bacterial diseases of fruit crops. Combinations of Theia® and strategic concentrations of two demethylation inhibitor (DMI) fungicides were investigated to explore potential synergisms. Bacteria were cultured in nutrient broth and combined with technical grades and two formulations of propiconazole (EC and WP) and metconazole (EC and WDG) at 0, 10, 50, 100, and 150 µg/ml active ingredient. After co-cultivation, optical density (OD600) and colony forming units (CFU/ml) were evaluated. In contrast to EC formulations, the WP or WDG formulations at 10 or 50 µg/ml of both DMIs did not affect vegetative cell growth. The mixture of Theia® and each formulated DMI at 50 µg/ml active ingredient resulted in a significant reduction of Monilinia fructicola lesion development on apple, Colletotrichum siamense lesion development on cherry, and Botrytis cinerea lesion development on cherry. The combination of Theia® with EC formulations showed weaker disease reduction due to antagonism. Only Theia® + non-EC formulated propiconazole and metconazole significantly reduced brown rot disease incidence of apple compared to the respective solo treatments and anthracnose disease incidence of cherry compared to the untreated control. Our results indicated that at least some DMI fungicides possess bactericidal effects depending on the formulation and concentration. The combination of Theia® with a lower than label rate concentration (50 µg/ml) of the DMI fungicides propiconazole and metconazole showed potential for synergistic effects especially when non-EC formulations were used.