Fire blight, caused by Erwinia amylovora , remains a major threat to apple production in Washington State. Although most infections occur during bloom, the post-bloom period also poses a high risk because actively growing shoots can be infected following injury caused by wind, rain, or hail. This study evaluated the efficacy of the systemic acquired resistance (SAR) inducer acibenzolar- S -methyl (ASM) and the plant growth regulator (PGR) prohexadione-calcium (ProCa) in managing shoot blight in young ‘WA 38’ apple trees. Field trials conducted in 2022, 2023, and 2024 assessed disease incidence and severity, and quantified the expression of six defense-related genes ( PR1 , PR2 , PR4 , PR14 , JAA , and NIMIN2 ) to determine plant defense activation. ProCa consistently reduced shoot blight incidence and severity, and showed a strong and sustained upregulation of genes related to plant defense responses. The combined treatment, consisting of low-doses of ASM and ProCa, demonstrated additive effects, enhancing both the expression of genes related to defense pathways and disease control. These findings suggest that ProCa, alone or in combination with ASM, is a promising tool for integrated shoot blight management in ‘WA 38’ apple trees. • ProCa reduced shoot blight in ‘WA 38’ apples, with high dose effective across seasons. • ProCa control was linked to activation of multiple plant defense-related genes. • Defense activation shows ProCa field efficacy goes beyond cell-wall thickening effects. • Combined treatment enhanced defense gene expression and reduced shoot blight severity.
Baró et al. (Fri,) studied this question.