Management of blossom blight in nectarine and peach with Bacillus subtilis.

Monilinia fructicola, the causal agent of blossom blight and brown rot in peaches and nectarines, is currently managed with chemical fungicide programs in the Southeastern United States. However, increasing fungicide resistance and regulatory pressure to reduce synthetic fungicide use highlight the need for alternative strategies. This two-year field study evaluated the efficacy of the biologicals Bacillus subtilis AFS032321 formulated as Theia and Pseudomonas chlororaphis AFS009 formulated as Howler EVO alone and in combination, for blossom blight control in nectarines at the Musser Fruit Research Center in Seneca, SC. In addition, Theia was compared to the commercial standards at three different farms in five cultivars. Theia significantly reduced blossom blight in both years with an observed control efficacy of 79.2% in 2025 and 57.3% in 2024 and was statistically equivalent to the control efficacy of 64.9% for synthetic fungicide propiconazole (Propi-Star EC) in 2025. In contrast, Howler EVO had no significant effect on disease reduction. Notably, the combination of the two biologicals in field trials at the experiment station resulted in antagonistic interactions. On nutrient agar, B. subtilis AFS032321 colony formation was inhibited by Howler EVO even at 1/10 of the label rate, supporting the field observation of antagonism between the two biologicals. No significant differences were found between Theia and the commercial standard programs for blossom blight control in commercial farm field trials. These findings demonstrate the potential but also highlight the vulnerability of biological use for blossom blight management in nectarines and peaches.