Abstract
Stemphylium leaf spot, caused by Stemphylium botryosum f. sp. spinacia, is an important fungal disease of spinach (Spinacia oleracea L.). The aim of this study was to conduct association analysis to identify single nucleotide polymorphism (SNP) markers associated with Stemphylium leaf spot resistance in spinach. A total of 273 spinach genotypes, including 265 accessions from the USDA spinach germplasm collection and eight commercial cultivars, were used in this study. Phenotyping for Stemphylium leaf spot resistance was evaluated in greenhouse; genotyping was conducted using genotyping by sequencing (GBS) with 787 SNPs; and single marker regression, general linear model, and mixed linear model were used for association analysis of Stemphylium leaf spot. Spinach genotypes showed a skewed distribution for Stemphylium leaf spot resistance, with a range from 0.2% to 23.5% disease severity, suggesting that Stemphylium leaf spot resistance in spinach is a complex, quantitative trait. Association analysis indicated that eight SNP markers, AYZV02052595_115, AYZV02052595_122, AYZV02057770_10404, AYZV02129827_205, AYZV0-2152692_182, AYZV02180153_337, AYZV02225889_197, and AYZV02258563_213 were strongly associated with Stemphylium leaf spot resistance, with a Log of the Odds (LOD) of 2.5 or above. The SNP markers may provide a tool to select for Stemphylium leaf spot resistance in spinach breeding programs through marker-assisted selection (MAS).
Highlights
Spinach (Spinacia oleracea L.) is an economically important vegetable crop worldwide [1] [2]
The distribution of Stemphylium leaf spot resistance was skewed to low disease severity and near normal after transformation by square root (Figure 1), indicating that Stemphylium leaf spot resistance is quantitative and probably controlled by multiple minor genes
A Q-value = 0.55 was used to divide the clusters, i.e., if a spinach genotype had a Q1 >= 0.55, the genotype was placed into Cluster Q1; if a spinach had a Q2 ≥ 0.55, it was placed into Cluster Q2; and the remaining genotypes (0.45 < Q1 < 0.55 or 0.45 < Q2 < 0.55) were placed into Q1Q2 of the admixture
Summary
Spinach (Spinacia oleracea L.) is an economically important vegetable crop worldwide [1] [2]. In 1997, a new leaf spot disease was discovered in limited spinach acreage in the Salinas Valley of California [5]. Spots on spinach leaves were 2 to 5 mm in diameter, circular to oval, and gray-green in color. These spots expanded and turned tan in color over time. The continued occurrence of the disease in spinach crops in the Salinas Valley, especially during rainy seasons, indicates that the pathogen has become established in that state. Quality standards for fresh market spinach are extremely high, so this disease poses an additional threat for growers in California and other states who need to produce defect-free products
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