Identification of a Causal Pathogen of Watermelon Powdery Mildew in Korea and Development of a Genetic Linkage Marker for Resistance in Watermelon (Citrullus lanatus)

Watermelon production is often limited by powdery mildew in areas with a large dailytemperature range. Development of resistant watermelon cultivars can protect against powderymildew; however, little is known about the characteristics of its causal agents. Here, weidentified the genus and race of a causal pathogen of powdery mildew in Ansung provinceof South Korea, and developed molecular markers for the generation of resistant watermeloncultivars. The causal pathogen was determined to be Podosphaera xanthii based on multiplesequence alignments of internal transcribed spacers (ITS) of rDNA. The physiological racewas identified as 1W, and the Ansung isolate was named P. xanthii 1W-AN. Followinginoculation with the identified P. xanthii 1W-AN, we found inheritance of the resistant genefitting a single dominant Mendelian model in a segregated population (‘SBA’ × PI 254744). Todevelop molecular markers linked to fungus-resistant loci, random amplified polymorphic DNA(RAPD) was accomplished between DNA pooled from eight near-isogenic lines (NILs; BC4F6),originated from PI 254744 and susceptible ‘SBB’ watermelon. After sequencing bands fromRAPD were identified in all eight NILs and PI254744, 42 sequence-characterized amplifiedregion(SCAR) markers were developed. Overall, 107 F2 plants derived from BC4F6 NIL-1 × ‘SBB’ were tested, and one SCAR marker was selected. Sequence comparison betweenthe SCAR marker and the reference watermelon genome identified three Nco I restrictionenzyme sites harboring a single nucleotide polymorphism, and codominant cleavage-amplifiedpolymorphic site markers were subsequently developed. A CAPS marker was converted to ahigh-resolution melt (HRM) marker, which can discriminate C/T SNP (254PMR-HRM3). The254PMR-HRM3 marker was evaluated in 138 F2:3 plants of a segregating population (‘SBA’× PI254744) and was presumed to be 4.3 cM from the resistance locus. These results couldensure P. xanthii 1W-AN resistance in watermelon germplasm and aid watermelon cultivardevelopment in marker-assist breeding programs. KCI Citation Count: 9