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Conservation of Antibiotic Efficacy in Management of Fire Blight of Pear and Apple

Stockwell, Virginia
Oregon State University
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  1. Determine prevalence of oxytetracycline resistance in indigenous bacteria in commercial pome fruit orchards. Commercial orchards in the western states have been treated with oxytetracycline and/or streptomycin for several years for control of fire blight. We will take samples from orchards to provide an estimate on the prevalence of oxytetracycline-resistant bacteria. Acquisition of oxytetracycline resistance by the fire blight pathogen E. amylovora will make antibiotics less effective for disease control. The risk of E. amylovora developing resistance to oxytetracycline from indigenous bacteria on flowers will be addressed in Obj 2.
  2. Determine potential for resistance of Erwinia amylovora to oxytetracycline and antibiotics proposed for fire blight control.
    Subobjective 2A. Evaluate probability of transfer of oxytetracycline resistance, stability, and fitness of oxytetracycline-resistant strains of Erwinia amylovora. Acquired resistance of E. amylovora to oxytetracycline has been shown in laboratory experiments, but the significance of this to commercial agriculture is unclear. We will introduce genes for oxytetracycline resistance and determine the stability of resistance and the fitness and virulence of the resistant pathogen. The results of this Objective will provide information critical to the assessment of risk of oxytetracycline resistance in E. amylovora in orchards.
    Subobjective 2B. Evaluate potential for resistance of Erwinia amylovora by spontaneous mutation to oxytetracycline and antibiotics proposed for fire blight control. If oxytetracycline resistance develops in E. amylovora in orchards, then growers will need alternative chemicals against fire blight. Several antibiotics are being considered in the US but little is known about the rate of mutation to resistance to these compounds by E. amylovora. Understanding the potential for spontaneous mutation conferring resistance to an antibiotic is important to assess the usefulness and expected lifespan of a new management tool against fire blight.
  3. Objective 3. Determine if integrated biologically-based fire blight management strategies mitigate the risk of development of populations of antibiotic-resistant indigenous bacteria. We will compare integrated pest management strategies with the conventional antibiotic resistance management strategy of spraying mixtures of antibiotics. We hypothesize that biocontrol bacteria used for fire blight will suppress the growth of populations of indigenous bacteria, including antibiotic resistant isolates, on pear and apple flowers. We will evaluate the commercially available biocontrol agents Pseudomonas fluorescens strain A506 (BlightBan A506) and Pantoea agglomerans strain C9-1S (BlightBan C9-1) and antibiotic sprays on growth of indigenous bacteria in orchards. We anticipate that these biocontrol agents will exclude a indigenous bacteria, including those resistant to oxytetracycline, from flowers. We may establish a new role for biocontrol agents in integrated pest management; as an important tool in the conservation of the efficacy of antibiotics.
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Non-Technical Summary: Fire blight caused by Erwinia amylovora is the most destructive bacterial disease to pear and apple orchards. The antibiotic streptomycin controlled fire blight until the pathogen gained resistance. Growers now rely on multiple sprays of oxytetracycline, but strategies to prevent resistance to this chemical are not understood. We found that integrated disease management, biocontrols followed by a single oxytetracycline spray, provides superior disease control. The sustainability of this approach is dependent on deterring pathogen resistance to oxytetracycline. The purpose of this study is to i) evaluate the risk of the fire blight pathogen developing resistance to oxytetracycline, ii) determine if the fire blight pathogen can acquire oxytetracycline resistance from native bacteria in orchards, and iii) determine if biological control mitigates the risk of oxytetracycline resistance by controlling growth of native bacteria. We hypothesize that an integrated disease management strategy (biocontrol agents followed by a single antibiotic treatment) will diminish the risk of antibiotic resistance by i) reducing the number of antibiotic sprays needed for disease control and ii) reduce populations of native bacteria that carry transferable antibiotic resistance genes. Biological suppression of antibiotic-resistant bacteria in orchards may mitigate concerns about agricultural use of antibiotics for plant disease control as a public health concern.

Approach: Fire blight, caused by Erwinia amylovora, is the most destructive bacterial disease of pear and apple. Streptomycin sprays controlled the disease until resistant pathogens become prevalent in the western states. These growers now rely on multiple sprays of oxytetracycline or biological control, both methods provide moderate or inconsistent control of fire blight. We found that integrated disease management, biocontrols followed by a single oxytetracycline spray, provides superior disease control. Sustainability of this method is dependent on continued registration of oxytetracycline and sensitivity of the pathogen to the antibiotic. Resistance to oxytetracycline is likely to arise by the fire blight pathogen acquiring genes from native bacteria on flowers. We will characterize antibiotic resistance of native bacteria in commercial pear and apple orchards. This includes determining which antibiotic resistance genes are present and if they can be transferred and maintained in the fire blight pathogen. The fitness and virulence of the antibiotic-resistant pathogen will be assessed as risk factors for development of resistance to. In case the fire blight pathogen gains resistance to oxytetracycline in orchards, we will evaluate the potential for resistance to proposed antibiotics for fire blight management. Native bacteria with resistance to oxytetracycline are often resistant to streptomycin, so combining antibiotics is unlikely to control native bacteria that are potential sources of antibiotic resistance to the pathogen. Our research will examine the potential of the bacterial biocontrol agents (Pseudomonas fluorescens A506 and Pantoea agglomerans C9-1) to suppress native bacteria harboring antibiotic resistance genes. In orchard trials, we will compare the impact of antibiotic sprays, biocontrols, and combinations of biocontrol agents followed by a single application of oxytetracycline on fire blight control and on populations of native bacteria, especially those with transferable antibiotic resistance. We anticipate that establishment of biocontrol bacteria in an orchard will suppress growth of native bacteria and also the fire blight pathogen; which will decrease the probability of the pathogen acquiring resistance to oxytetracycline. We hypothesize that the integrated biological control strategy can diminish the rate of antibiotic resistance in two ways: 1) biological control is expected to reduce the need for antibiotic applications, thus decreasing selection pressure for antibiotic-resistant bacteria and 2) biological control is expected to reduce populations of native bacteria, which can be sources of antibiotic resistance genes that may be transferred to the fire blight pathogen and other bacteria. An expected outcome of the proposed research is the development of methods that provide consistent and sustainable management of fire blight, a bacterial disease that poses a continuing threat to pear and apple industries. The judicious integration of antibiotics with biological control agents also may mitigate concerns about pesticide use of antibiotics as a public health concern.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Sanitation and Quality Standards
Risk Assessment, Management, and Communication
Bacterial Pathogens