Integration of acibenzolar-S-methyl with antibiotics for protection of pear and apple from fire blight caused by Erwinia amylovora

Orchard experiments on integration of acibenzolar-S-methyl (ASM) with antibiotics for protection of pear and apple from fire blight were conducted in the west coast region of the United States over a period of 5 years. In 11 pathogen-inoculated trials, a single treatment of streptomycin or oxytetracycline provided an average of 84 and 60% disease control, respectively. The addition of one or two treatments of acibenzolar-S-methyl (ASM) to the single antibiotic program contributed an additional 6 and 11% disease control, respectively, for both antibiotic materials. Among trials, ASM treatment timings were varied from early to late bloom but an effect of timing on disease control could not be determined. In mature commercial pear orchards, ASM treatments at full bloom and petal fall were superimposed onto the grower’s antibiotic program used in each orchard. For the 15 orchards with fire blight, the ASM-treated plots showed 38% fewer infections than adjoining plots that received antibiotic program only. When integrated with antibiotics, ASM provides added disease suppression to fire blight control programs, but the modest degree of protection provided will likely limit its use to high disease risk situations, which includes orchards with a previous disease history, and those planted recently to highly susceptible cultivars.