39 Overview of Concerns Surrounding Antibiotic Use for Control of Fire Blight V.O. Stockwell Department of Botany and Plant Pathology Oregon State University Corvallis, Oregon 97331 USA Keywords: Erwinia amylovora, streptomycin, tetracycline, pesticide residue, antibiotic resistance Abstract The antibiotic streptomycin was first used for control of fire blight of pear and apple in the 1950s and quickly became an important tool for management of this serious bacterial disease. Since the beginning of antibiotic use on plants, questions were raised over the potential for development of resistance to antibiotics by the plant pathogen Erwinia amylovora and unintended detrimental effects on the environment and human health. To reduce selection pressure for antibiotic resistant populations of E. amylovora, the number of applications are few and applied when predicted disease risk is high. Studies to address persistence of antibiotics in the environment demonstrated that antibiotics are active on plant surfaces for less than a week and are rapidly inactivated in soils. Residues exceeding permissible levels set by regulatory agencies have not been reported on harvested fruit. Metagenomic studies found that within a month after an antibiotic application, bacterial communities in orchards treated with antibiotics were not significantly different from those from non-treated orchards. Clinical bacteria were not found as resident microbes in the orchard phyllosphere and a direct link between antibiotic use in orchards and antibiotic resistance in human pathogens has not been demonstrated. Overall, antibiotics have been indispensable for fire blight management for more than 60 years without reports of adverse effects on human health or persistent deleterious effects on the environment. INTRODUCTION Streptomycin has been an important tool for control of fire blight since the 1950s. Numerous antibiotics are used in clinical medicine or animal agriculture, but only five antibiotics are used on plants. Of the antibiotics used on plants, streptomycin is used most widely. The sustainability of antibiotics for plant disease prevention has been threatened by emergence of streptomycin-resistant populations of Erwinia amylovora, and challenges to the registration of antibiotics for fire blight control. This brief review will cover the current use of antibiotics in orchards and highlight research that addresses some of the persistent questions about the safety of antibiotic use in orchards. DISCUSSION Antibiotics Used for Control of Fire Blight Three of the five antibiotics used in apple and pear orchards in various countries are aminoglycosides. The most widely used aminoglycoside in plant agriculture is streptomycin. Streptomycin is registered for control of fire blight, caused by Erwinia amylovora, in Canada, Israel, Mexico, New Zealand and the USA; it has been permitted on an emergency use basis, subject to annual review and under tightly restricted conditions, in Austria, Germany and Switzerland (McManus et al., 2002; Stockwell and Duffy, 2012). Another aminoglycoside antibiotic gentamicin is used in Mexico against fire blight and other diseases. The aminoglycoside kasugamycin, which has not been used clinically, is under consideration for registration in the USA. Oxolinic acid, a synthetic Proc. XIII th International Workshop on Fire Blight Eds.: F. Rezzonico, T.H.M. Smits and E. Holliger Acta Hort. 1056, ISHS 2014