Evaluation of Biorational Fungicides Against Peach Scab in Field Trials

Scab in conventional peach orchards in the southeastern United States is managed with the synthetic fungicide chlorothalonil. Due to increasing Environmental Protection Agency (EPA) restrictions, especially for multisite fungicides and growing demand for more sustainable integrated pest management (IPM) approaches, there is interest in finding safer alternatives. In this study, the efficacy of three biorational fungicides—Howler EVO ( Pseudomonas chlororaphis strain AFS009 metabolites), Theia ( Bacillus subtilis strain AFS032321), and CrystalPro (calcium propionate)—was investigated for peach scab management. Field trials conducted on cultivar ‘Juneprince’ in 2022 to 2024 and ‘Sweet Dream’ in 2025 revealed that Howler EVO and Theia had no effect on peach scab disease incidence or severity. Likewise, CrystalPro at 0.1 and 0.8% and pH 5.1 to 5.7 had no effect against peach scab incidence except for CrystalPro at 0.8% in year 2022. CrystalPro 0.1 and 0.8% reduced scab severity in one and two of three experimental years, respectively. Chlorothalonil (Bravo Weather Stik) was the best treatment and consistently reduced peach scab incidence and severity across all experimental years compared with the untreated control. In conclusion, none of the biorational fungicides tested in this study can substitute the synthetic standard without risking higher disease incidence and severity; however, CrystalPro at 0.8% may be useful for organic growers to reduce the number of scab lesions on peach fruit.