*2.1. Plant Materials and Genomic DNA Extraction*

To develop molecular markers, four (S&P 7522, S&P 7521, S&P 7129, and S&P 7168) gray mold-resistant and three (S&P 7130, S&P 7175, and S&P 7483) susceptible onion (*Allium cepa* L.) lines grown in 'Seeds & People' Co., Ltd. (Yeonggwang, Korea) were used in this study. The characteristics of the seven onion lines of 'Seeds & People' Co. are shown in Table S1. The pedigree of the seven onion lines used in this study is shown in Figure S1. In addition, to confirm the versatility of the developed molecular markers, three (Asia-12, Asia-42, and Asia-53) resistant and four (Asia-30, Asia-35, Asia-45, and Asia-50) susceptible onion lines against gray mold were provided by 'Asia Seed' Co., Ltd. (Seoul, Korea) and analyzed. Genomic DNA was isolated from the leaf tissues of each line by

using cetyltrimethylammonium bromide (CTAB) [21]. The concentration and purity of the gDNA were measured using a Nanodrop® ND-1000 spectrophotometer (NanoDrop Technologies, Wilmington, NC, USA).
