Date of Graduation


Document Type


Degree Name

Master of Science in Horticulture (MS)

Degree Level





Margaret Worthington

Committee Member

John Clark

Second Committee Member

Richard Mason

Third Committee Member

Clemencia Rojas


Bacteria, Genetics, Peach, QTL, resistance, Xanthomonas


Bacterial spot, caused by the bacterium Xanthomonas arboricola pv. pruni (Xap), is a threat to the peach [Prunus persica (L.) Batsch], Japanese and European plum (P. salicina L. and P. domestica L.), and tart and sweet cherry (P. cerasus L. and P. avium L.) industries. Markers for fruit resistance to bacterial spot have been developed however, markers associated with foliar resistance have yet to be developed. A total of 130 progeny and 13 parents (n=143) were evaluated for foliar and fruit Xap resistance in 2013, 2014, and 2015, and 162 progeny and eight parents (n=170) in 2017 and 2018 as part of a genome-wide association study (GWAS). Individuals were genotyped with the peach 9K or 9+9K genotyping array. Foliar resistance was quantitatively inherited and controlled by several small-effect QTLs located on chromosomes 1, 2, 3, 5, 6, and 7. Fruit resistance was also quantitatively inherited and controlled by multiple small-effect QTLs located on chromosomes 1, 2, and 3. Xap has been documented to have low genetic diversity compared to other X. arboricola pathovars. Even with potentially low genetic diversity, it is thought that there are substantial genetic differences in Xap populations and their degree of virulence. Previous studies were limited due to a low number of isolates from a small number of locations. In this study 673 Xap isolates were collected from 12 states across the United States, to investigate the diversity and virulence of strains from different locations. Eighteen simple sequence repeats (SSR) with di-, tri- or tetra-nucleotide repeats were identified within the Xap CFBP5530 plasmid pXap41 and used to screen Xap isolates. A detached leaf assay was performed on Xap positive isolates to confirm pathogenicity and record virulence. Pathogenicity among all isolates was confirmed, but virulence levels were low. Xap strains were grouped based on marker scores of SSR loci using GenAlEx 6.5 and two dendrograms were generated using the R package “Poppr”. The unweighted pair group with arithmetic mean (UPGMA) dendrogram analysis showed two major clusters. The neighbor joining (NJ) dendrogram analysis showed three major clusters. Genetic diversity was observed in the United States Xap collection.