Resistance of garden roses to cercospora leaf spot

Abstract

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...

Occurrence of Diseases in Floricultural Crops in and around Jorhat, Assam

“…The conidia were observed under microscope as cylindrical and having septa ( Figure 26) ( Table 3). The results were in conformity with the study of Kang et al (2019). Figure 27).…”

Pedigree-based analysis in multi-parental diploid rose populations reveals QTLs for cercospora leaf spot disease resistance

“…In this study, CLS showed low to moderately high h 2 and moderately high H 2 , as previously reported (Kang et al, 2019). The G×E/G ratios (1.33 to 2.33) and GGE biplots indicated that rose genotypes exhibited different patterns of CLS incidence in different environments (location and/or month/year).…”

“…Using PBA and two diploid rose multiparent populations, 38 QTLs distributed over the seven LGs were identified for CLS resistance, consistent with reports indicating that CLS resistance is polygenic in rose (Kang et al, 2019). Three large-effect QTLs associated with CLS resistance in each population were consistently detected with decisive evidence in most data sets on LGs 3, 4, and 6.…”

“…The G×E/G ratios (1.33 to 2.33) and GGE biplots indicated that rose genotypes exhibited different patterns of CLS incidence in different environments (location and/or month/year). Reason for G×E interactions appeared to be the low levels and non-uniform distribution of CLS incidence likely due to unfavorable weather conditions (hot and dry) and/or low initial levels of inoculum, which decreased our ability to distinguish between susceptible and resistant genotypes, as previously reported (Kang et al, 2019). In TX2WOB, the presence of G×E in this population may be attributed to the CS 2016 environment as the total precipitation in this location/year combination was higher than SV 2019 and SV 2021 (~1121 mm vs. 823 and 830 mm, respectively) along with higher relative humidity (Supplementary Table 6).…”

“…CLS resistance in roses (Rosa ssp.) is reported to be quantitatively inherited with a low to moderate narrow-sense (h 2 = 0.51) and moderate to high broad-sense heritabilities (H 2 = 0.72) (Kang et al, 2019), indicating that resistance should be a feasible breeding goal. Thus far, no distinct races of CLS or resistance genes have been identified for CLS, whereas genes, QTLs, and pathogenic races associated with BSD (Soufflet-Freslon et al, 2019;Yan et al, 2019;Lopez Arias et al, 2020a;Lopez Arias et al, 2020b) and powdery mildew (Xu et al, 2005;Linde et al, 2006;Xu et al, 2007;Hosseini Moghaddam et al, 2012;Leus et al, 2015) have been reported.…”