During apple ( Malus × domestica Borkh.) orchard establishment, fire blight outbreaks caused by Erwinia amylovora cause significant losses. The use of streptomycin is common; however, increased antibiotic resistance and public concern regarding pesticide safety necessitate alternative disease management solutions. Prohexadione-calcium (P-Ca) is a plant growth regulator that inhibits shoot growth and thickens cell walls, effectively reducing the penetration of bacteria into cells. Acibenzolar- S -methyl (ASM) is a systemic acquired resistance inducer that primes the immune system of a plant to allow for better defense when exposed to a pathogenic microorganism. The objective of this study was to determine the main effects and interactions of P-Ca and ASM on disease incidence and severity and bacterial population density in tissues with varying distances from the lesion edge. In 2022 and 2023, the study was conducted in a ‘Gala’ orchard in its second leaf and third leaf. Treatments were applied in a factorial treatment structure of two factors: 0, 42.5, and 125 mg·L −1 for P-Ca and 0 and 37.5 mg·L −1 for ASM. At the beginning of each season in 2021 to 2023, P-Ca and ASM were applied twice to the same plots each year. Shoot inoculations with E. amylovora occurred on 12 May 2022 and 11 May 2023. In 2022 and 2023, the P-Ca rate had a negative curvilinear relationship with disease incidence and severity across multiple dates. In 2022, ASM was variably impactful and interacted with P-Ca to lessen disease symptoms; however, this did not occur in 2023. On the final sampling date in 2023, bacterial density in the scion was lessened as the rate of P-Ca increased.
Vogel et al. (Wed,) studied this question.