Date of Award

August 2020

Document Type


Degree Name

Master of Science (MS)


Division of Agriculture (SAFES)

Committee Member

Guido Schnabel

Committee Member

Hehe Wang

Committee Member

Juan Carlos Melgar


Bacterial spot on peach, caused by Xanthomonas arboricola pv. pruni (Xap), is a major disease in the southeastern United States. The disease can cause substantial yield loss despite season long applications of copper. It is unknown whether selection of resistance over the course of the season contributes to disease development. Thus, we collect Xap from shoot cankers, leaves, and fruit over two years from cultivar O‘Henry of three conventional and one organic farms in South Carolina and determined sensitivity to copper at the beginning (bud break), middle (pit hardening) and end (final swell) of production season. Four canker types were identified in both years, including bud cankers (infected flowering or leaf bud), tip cankers (necrotic tip of one year old shoot), concentric cankers (classic oval-shaped canker on one year shoot), and non-concentric cankers. Xap isolation rate was dependent on farm and canker type; more Xap were successfully isolated from the organic farm (24% of the canker) compared to two of the conventional farms and most (45%) came from bud cankers. Xap isolates were assessed for sensitivity to copper using two types of media, minimal glucose yeast agar and nutrient agar, amended with Copper sulfate pentahydrate (CuSO4∙5H2O) at the discriminatory dose for tolerance at 150 µg/ml and for resistance at 200 µg/ml. In this study two phenotypes of copper tolerant Xap strains were discovered low copper tolerant (grew up to 150 µg/ml) and high copper tolerant (grew up to 200 µg/ml). Regardless of the farm and collection year, most Xap strains were sensitive to copper but tolerance was observed in 58 out of 298 strains. A total of 26 out of 139 and 32 out of 101 from the 2018 and 2019 collection, respectively, were tolerant to copper. The study illuminates shoot canker types and phenotypic diversity among bacterial populations within and between farms and assesses the importance of copper tolerance phenotypes to the success of chemical management programs.



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