STUDIES ON THE EPIDEMIOLOGY AND MANAGEMENT OF BACTERIAL SPOT OF PEACH AND NECTARINE IN PENNSYLVANIA

Open Access
Author:
Capasso, Sarah Jane
Graduate Program:
Plant Pathology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 08, 2010
Committee Members:
  • Henry K Ngugi, Thesis Advisor
Keywords:
  • Xanthomonas arboricola pv. pruni
  • survival analysis
  • disease assessment
  • bacterial spot
  • rating scale
  • peach
  • epidemiology
  • agreement study
Abstract:
The purpose of this research was to develop methods to quantify bacterial spot severity in order to improve the quality of data collected as well as to gain a better understanding of the epidemiology of bacterial spot (caused by Xanthomonas arboricola pv. pruni) in Pennsylvania peach and nectarine orchards. Based on Lin’s concordance analysis as well as the categorical data analysis, the direct estimation method was a superior method of estimating bacterial spot severity compared to a 1 to 7 ordinal rating scale (1 = 0% lesion area, and 7 = >45%). Moreover, the reliability and accuracy of estimates of bacterial spot severity made by inexperienced raters using the direct estimation method were equal to those made by experienced raters. In both 2008 and 2009, cultivar and bactericide treatment were the significant factors affecting the rate of bacterial spot progress based on analysis of 84 temporal disease progress curves from each year. Survival analysis of data collected in 2009 showed that the mean time to leave abscission (T) ranged from 41.8 to 56.3 days, and was significantly (P < 0.0001) affected by cultivar, initial disease severity at the onset of the epidemic, and leaf age, but not by bactericide treatment. Results indicate that bacterial spot epidemics do not follow standard disease progress curves, and that strategies for bacterial spot management should focus on reducing initial disease. In addition, estimates of bacterial spot severity can be made by inexperienced raters without loss of accuracy and reliability with the use of the direct estimation method. The results are discussed in relation to their implication to bacterial spot management in the eastern US.