Antibiotic Resistance in Pennsylvania Stone Fruit Orchards

Open Access
Capasso, Sarah Jane
Graduate Program:
Plant Pathology
Doctor of Philosophy
Document Type:
Date of Defense:
November 23, 2015
Committee Members:
  • Maria Del Mar Jimenez Gasco, Dissertation Advisor
  • Beth Krueger Gugino, Committee Member
  • Gary William Moorman, Committee Member
  • Kari Anne Peter, Committee Member
  • Maryann Victoria Bruns, Committee Member
  • antibiotic resistance
  • tetracycline
  • antibiotics
  • oxytetracycline
  • peach
  • nectarine
  • orchard
  • Pennsylvania
  • tree fruit
Bacterial spot (caused by Xanthomonas arboricola pv. pruni) is the most important bacterial disease of peach and nectarine in the eastern United States. The antibiotic oxytetracycline is used to mitigate the yield limiting symptoms of this disease. Despite that, yield loss remains high in susceptible stone fruit cultivars, raising concern among growers over the development of antibiotic resistance in the causal pathogen. Previous surveys of the stone fruit orchard bacterial community indicated the presence of oxytetracycline resistant epiphytic bacteria. This was significant because epiphytic or nontarget bacteria are thought to harbor more resistance genes and often before coexisting pathogens. Under a strong selection pressure such as repeated antibiotic applications, transfer of resistance genes from epiphytic bacteria to pathogenic bacteria is favored. Therefore, when evaluating antibiotic resistance development in pathogenic bacteria, nontarget bacteria must also be considered. The overall goal of this research was to determine the consequences of repeated oxytetracycline applications in commercial Pennsylvania stone fruit orchards, including management factors related to the incidence of bacteria carrying tetracycline resistance genes and the sensitivity of X. arboricola pv. pruni isolates to oxytetracycline. Tetracycline resistance genes, tet(A), tet(B), and tet(C), were found in epiphytic bacteria recovered from commercial stone fruit orchards and research blocks at the PSU Fruit Research and Extension Center. The most common carriers of these resistance genes were bacteria that belonged to the genera Pantoea and Pseudomonas where tet(B) was most commonly associated with the former and tet(C), the latter. Bacteria carrying these tetracycline resistance genes could grow on media amended with greater than 450 ug/ml of oxytetracycline, three times that of the rate used in the field to manage bacterial spot. While the incidence of tetracycline resistance genes in epiphytic bacteria significantly differed (P > 0.0001) among the sampled commercial orchards, this was not related to oxytetracycline use (P = 0.0855). When tested in the experimental stone fruit blocks, again, the distribution of bacteria positive for tetracycline resistance genes was not related to bactericide treatment (2013: P = 0.407; 2014: P = 0.520). Other management factors including tree age, cultivar, and sampling date were. Sensitivity to oxytetracycline among Xap isolates significantly differed (P > 0.0001) among those collected from commercial stone fruit orchards. While no tetracycline resistance genes were found in any of the sampled Xap isolates and overall sensitivity remained high (MIC < 25 µg/ml), oxytetracycline use was a significant factor associated with oxytetracycline sensitivity (P > 0.0001). Further research should be conducted to determine the molecular mechanism associated with the variability in sensitivity in the Xap isolates, including novel and untested resistance genes, adaptive resistance, or mutation.