Cercospora rosicola is a fungal pathogen that attacks roses and causes spotting on leaves, chlorosis and in severe cases, defoliation. This disease has become more prominent in roses after the widespread use of black spot resistant roses, most likely due to the reduction of fungicide applications on roses. The objective of this thesis is to evaluate garden roses and identify QTLs for resistance to cercospora leaf spot disease. Identification of susceptible and resistant roses to cercospora leaf spot is the first step for breeding cercospora resistant rose cultivars. Cercospora leaf spot was evaluated on a percentage-based rating scale of 0-9 (0=no disease and 9=foliage covered with lesions) during spring, summer and fall of 2016 through 2018 on 130 roses in Overton and College Station, TX. Average cercospora incidence in Overton increased from 2016 to 2017, but decreased in 2018, whereas in College Station, ratings increased three-fold from 2016 (0.6) to 2018 (2.2). Most cultivars showed tolerance to the disease, particularly Rosa rugosa hybrids. However, a number of cultivars including 'American Pillar', 'John Davis', 'Carefree Delight', 'Oso Happy Candy Oh', 'Oso Easy Cherry Pie' and 'Roxanne Veranda' were highly susceptible to cercospora. In College Station, moderately high repeatability and low coefficient of variance occurred in the later months of 2017 and 2018, while in Overton, all the evaluated months in 2016 except for April showed high repeatability and low coefficient of variance. This suggests that ratings during these months are consistent, and may be the more informative months for disease evaluation. Artificial inoculation in a greenhouse setting was also attempted to identify cercospora leaf spot resistant garden roses. Although the pathogen was successfully cultured, the low sporulation, high heat and low humidity during the inoculation iii period slowed disease development. Thus, severe disease symptoms were not observed for this experiment.In 2016, fifteen diploid inter-related rose populations were evaluated for cercospora leaf spot in June, September, October and November in College Station. The estimated broad sense and narrow sense heritability were 0.83 and 0.57, respectively. A pedigree-based analysis using Visual FlexQTL software was conducted on these populations. QTLs found on LG1, LG3 andLG7 appeared in different environments. In the overall analysis, QTLs found on LG1 (0-4cM) and LG3 (36-42cM) explained 8.5% and 7.7% of the total phenotypic variance, respectively.More studies are needed to improve the strength and consistency of QTL detection in this analysis. iv DEDICATION This thesis is dedicated to my father, Yang Woo Kang and my mother, Soo Young Kim for their love and unconditional support. I am also thankful to my sisters Swanie, Serena and Sharon Kang for their patience and encouragement during times of stress. v 2006). Roses are highly heterozygous and prone to moderate inbreeding depression. The inbreeding between modern rose classes may have led to reduced fertility (Zlesa...
Cercospora leaf spot (CLS) (Cercospora rosicola) is a major fungal disease of roses (Rosa sp.) in the southeastern U.S. Developing CLS-resistant cultivars offers a potential solution to reduce pesticide use. Yet, no work has been performed on CLS resistance. This study aimed to identify QTLs and to characterize alleles for resistance to CLS. The study used pedigree-based QTL analysis to dissect the genetic basis of CLS resistance using two multi-parental diploid rose populations (TX2WOB and TX2WSE) evaluated across five years in two Texas locations. A total 38 QTLs were identified across both populations and distributed over all linkage groups. Three QTLs on LG3, LG4, and LG6 were consistently mapped over multiple environments. The LG3 QTL was mapped in a region between 18.9 and 27.8 Mbp on the Rosa chinensis genome assembly. This QTL explained 13 to 25% of phenotypic variance. The LG4 QTL detected in the TX2WOB population spanned a 35.2 to 39.7 Mbp region with phenotypic variance explained (PVE) up to 48%. The LG6 QTL detected in the TX2WSE population was localized to 17.9 to 33.6 Mbp interval with PVE up to 36%. Also, this study found multiple degrees of favorable allele effects (q-allele) associated with decreasing CLS at major loci. Ancestors ‘OB’, ‘Violette’, and PP-M4-4 were sources of resistance q-alleles. These results will aid breeders in parental selection to develop CLS-resistant rose cultivars. Ultimately, high throughput DNA tests that target major loci for CLS could be developed for routine use in a DNA-informed breeding program.
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