Impact of DMI fungicides on Bacillus subtilis cell growth and consequences for disease control

Strategic mixtures of biological control agents (BCA) with demethylation inhibitor (DMI) fungicides to control diseases of fruit crops can result in additive interaction, synergism, or antagonism. To evaluate the compatibility of eight commercial DMI fungicides with the BCA Bacillus subtilis AFS032321, marketed as Theia, bacteria were cultured in nutrient broth with 0, 10, 50, 100, and 150 μg/ml of each DMI fungicide at 30°C on a shaker rotating at 180 rpm, and the optical density was measured after 24 and 48 h. In addition, nutrient agar (NA) was enriched with the same concentrations, and an endospore suspension was streaked out to count colony-forming units (CFU/ml) and measure the colony diameter. Results showed vegetative cell growth was strongly inhibited at ≥50 μg/ml of difenoconazole and ≥100 μg/ml of tebuconazole. All other DMI fungicides had little impact on B. subtilis AFS032321 cell growth at any of the concentrations tested. Interestingly, mefentrifluconazole significantly promoted colony growth (diameter) at all concentrations on NA. Theia applied at the label rate combined with vegetative growth-promoting DMI fungicide mefentrifluconazole (formulated as Cevya) or vegetative growth-suppressing DMI fungicide difenoconazole (formulated as Inspire) at 150 μg/ml of active ingredient were investigated against gray mold of cherry. Disease incidence and severity 5 days after inoculation indicated the best control efficacy and synergism (+10.2) for the mixture Theia + Cevya. For the mixture Theia + Inspire, an antagonistic effect (-6.8) was calculated. Our results indicate that compatibility between biological and conventional fungicides must be considered if they are used in mixtures or together in integrated spray programs.