Date of Award

5-2026

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Plant and Environmental Science

Committee Chair/Advisor

Ksenija Gasic

Committee Member

Guido Schnabel

Committee Member

Juan Carlos Melgar

Abstract

Bacterial canker, caused by Pseudomonas syringae pv. syringae (Pss), is a devastating disease affecting over 180 species and is one of the threats to stone fruit production worldwide. South Carolina is the second-largest peach-producing state in the United States, with 38,000 tons of annual production valued at approximately $80 million, and experiences significant financial losses due to this disease. Pss enters the tree through injured areas (caused by frost, leaf abscission, stomata, lenticels etc.). It infects buds, flowers, leaves, and shoots, eventually spreading to branches, limbs, and trunks, and causing necrosis and cankers, which leads to the tree branches dying back from the tips towards the trunk, a condition known as dieback. Current management strategies are mainly focused on preventing the tree decline by reducing the effects of predisposing factors, such as orchard location and soil characteristics, rootstock choice, freezing injury, wounds, and nematode pressure. Chemical treatments are based mainly on protective copper sprays; however, copper sprays are only preventative and are not effective treatments after infection has occurred. In addition, copper-resistant strains of Pss have been previously identified, highlighting the need for sustainable and permanent solutions that focus not on external control measures, but rather on the development of genetically tolerant cultivars.

In this research, I used a detached dormant twig assay to investigate Pss response in 134 peach accessions, comprised of cultivars grown in South Carolina, important rootstocks and breeding selections. Disease severity responses were assessed in two independent experiments at six weeks after inoculation by utilizing a visual scale and ImageJ software. Two different assessment methods provided a strong and highly consistent correlation, allowing the use of either method to quantify lesion severities. Considerable variation was observed in lesion severity among peach accessions, allowing their classification into three clusters: tolerant, intermediate and susceptible, with 43, 42 and 49 accessions, respectively. Furthermore, the BLUEs, BLUPs and LSMean values were calculated to determine whether the random and fixed effects have a significant influence on genotype performance to Pss infection; however, no changes in clustering and ranking were detected between estimation values. This finding supports the robustness of phenotypic classification of cultivars into different potential tolerance groups and suggests the genetic background plays an important role in the response to Pss infection among different peach accessions. Broad-sense heritability (H2), estimated at 47%, further supported that genetic factors have a considerable influence on disease response, suggesting that breeding for Pss tolerance in peach should be feasible. The results of this study provide the first evidence of tolerance to Pss in peach germplasm and lay a foundation for further research into the genetic control of the Pss tolerance in peach.

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