Abstract
Powdery mildew (PM) is one of the most serious diseases in cucumber and causes huge yield loss. Multiple quantitative trait loci (QTLs) for PM resistance have been reported in previous studies using a limited number of cucumber accessions. In this study, a cucumber core germplasm (CG) consisting of 94 resequenced lines was evaluated for PM resistance in four trials across three years (2013, 2014, and 2016). These trials were performed on adult plants in the field with natural infection. Using genome-wide association study (GWAS), 13 loci (pmG1.1, pmG1.2, pmG2.1, pmG2.2, pmG3.1, pmG4.1, pmG4.2, pmG5.1, pmG5.2, pmG5.3, pmG5.4, pmG6.1, and pmG6.2) associated with PM resistance were detected on all chromosomes except for Chr.7. Among these loci, ten were mapped to chromosomal intervals where QTLs had been reported in previous studies, while, three (pmG2.1, pmG3.1, and pmG4.1) were novel. The loci of pmG2.1, pmG5.2, pmG5.3 showed stronger signal in four trials. Based on the annotation of homologous genes in Arabidopsis and pairwise LD correlation analysis, candidate genes located in the QTL intervals were predicted. SNPs in these candidate genes were analyzed between haplotypes of highly resistant (HR) and susceptible (HS) CG lines, which were defined based on combing disease index data of all trials. Furthermore, candidate genes (Csa5G622830 and CsGy5G015660) reported in previous studies for PM resistance and cucumber orthologues of several PM susceptibility (S) genes (PMR5, PMR-6, and MLO) that are colocalized with certain QTLs, were analyzed for their potential contribution to the QTL effect on both PM and DM in the CG population. This study shows that the CG germplasm is a very valuable resource carrying known and novel QTLs for both PM and DM resistance, which can be exploited in cucumber breeding.
Highlights
Cucumber (Cucumis sativus L.) is an important vegetable crop with a long history of cultivation
In our cucumber core germplasm (CG) germplasm, resistance to both Powdery mildew (PM) and DM resistance [34] was identified, which are associated with many genetic loci scattered all over the seven cucumber chromosomes
Some loci are located in chromosomal regions where quantitative trait locus (QTL) and plant S gene for PM and DM have been reported in previous studies
Summary
Cucumber (Cucumis sativus L.) is an important vegetable crop with a long history of cultivation. Powdery mildew (PM), caused mainly by the fungus Podosphaera fusca is one of the most important diseases in cucumber and occurs extensively in various countries all over the world [1,2,3,4]. At the early stage of the infection, white, small round powdery fungal colonies appear on the top of the leaves, which gradually expand, leading to the joining of individual colonies. Leaf areas underneath powdery colonies turn chlorotic and the infected leaves gradually shrink. The infected leaves lose photosynthetic function, leading to prematurely aged and dead. Due to the decrease on foliage and photosynthetic efficiency, yield from PM infected plants are reduced
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