Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 1/10/2008
Publication Date: 6/2/2008
Citation: Stockwell, V., Temple, T., Johnson, K., Loper, J.E. 2008. Integrated control of fire blight with antagonists and oxytetracycline. Acta Horticulturae. 793:383-390. Interpretive Summary: Fire blight is a devastating disease of pears, apples, and related ornamental and crop plants. In severe years, the disease can kill trees in commercial orchards, with ruinous economic consequences. Since the mid-1900s, fire blight has been controlled by spraying trees with antibiotics, especially the antibiotic streptomycin. These antibiotics are toxic to the fire blight pathogen, a bacterium called Erwinia amylovora. Unfortunately, due to the prevalent use of streptomycin in many commercial orchards, the pathogen is now resistant to streptomycin in many regions of the United States. Consequently, streptomycin is no longer effective in those regions, and other ways to control the disease are needed. The antibiotic oxytetracycline is now sprayed as an alternative to streptomycin. In our field experiments described here, oxytetracycline was less effective than streptomycin in controlling fire blight caused by strains of the pathogen that are sensitive to both antibiotics. In an effort to improve disease control, we tested combinations of biological control agents and oxytetracycline in eight orchard trials. Two sprays of streptomycin or oxytetracycline reduced the disease incidence by an average of 76% and 42%, respectively compared to water-treated controls. When used in combination, the biological control strains reduced disease by 42%. An integrated treatment, i.e., a spray of biological control agents followed by one application of oxytetracycline, provided 57% control. Biological and chemical methods of fire blight suppression appear to be complementary, and can be used effectively together to manage fire blight. The alternative disease management strategies are especially important in regions where the fire blight pathogen is resistant to streptomycin. Therefore, this research will help to maintain a pome fruit industry in regions where fire blight would otherwise make it very difficult to grow pears or apple varieties that are sensitive to this devastating disease.
Technical Abstract: In the Northwest United States, the antibiotic streptomycin provided excellent control of fire blight until resistant isolates of the pathogen arose. Oxytetracycline (Mycoshield) is now sprayed as an alternative antibiotic. We found that the durability of inhibitory activity of oxytetracycline is similar to that of streptomycin, but oxytetracycline is considerably less effective than streptomycin when the antibiotics are targeted toward sensitive strains. In an effort to improve disease control, we evaluated combinations of biological control agents (Pseudomonas fluorescens A506 or Pantoea agglomerans C9-1) and oxytetracycline in eight orchard trials inoculated with an antibiotic-sensitive strain of Erwinia amylovora. Two bloom sprays of streptomycin or oxytetracycline reduced the disease incidence by an average of 76% and 42%, respectively compared to water-treated controls. A combination of C9-1 and a protease-deficient A506 provided 42% disease control. An integrated treatment, i.e., a spray of biological control agents followed by one application of oxytetracycline, provided 57% control. Biological and chemical methods of fire blight suppression appear to be complementary, and consequently, an integrated strategy consisting of a biological control agent sprayed in early and near full-bloom, followed by oxytetracycline treatment at late bloom, improved disease control with a reduced number of antibiotic applications.