QTL Mapping and KASP Marker Development for Powdery Mildew Resistance in Watermelon from a Novel Source (PI 482278)

Powdery mildew, caused by Podosphaera xanthii, remains a major constraint in watermelon (Citrullus lanatus) production. Development of disease resistant cultivars is one of the best strategies to manage powdery mildew in watermelon. We investigated the genetic basis of powdery mildew resistance in the donor line USVL278-PMR (S 5 progeny derived from PI 482278) using segregating F 2 and backcross populations developed from a cross with the susceptible line USVL677-PMS. Disease evaluations under controlled and field conditions indicated a single, dominant locus conferring powdery mildew resistance. Microscopic assays revealed restricted hyphal growth and suppressed conidiophore formation on resistant tissues. Bulked segregant analysis coupled with whole-genome sequencing (BSA-seq) mapped a major 566 kb quantitative trait locus on chromosome 2, which was refined to a ~51 kb interval using Kompetitive Allele-Specific PCR (KASP) markers. KASP markers previously developed for other powdery mildew resistance sources (USVL531-MDR and USVL608-PMR) from the same chromosomal region did not distinguish the parental lines (USVL278-PMR); therefore, new sets of KASP markers were developed and validated across F 2 (n = 434) and reciprocal backcross (n = 100) populations. The lipoxygenase (LOX, ClCG02G015730) gene based USVL-PMR-4 KASP marker exhibited 100% linkage with the phenotype (R 2 = 0.89), facilitating efficient marker-assisted selection. Results of this study can help accelerate transfer of powdery mildew resistant gene from USVL278-PMR to elite germplasm and help the development of powdery mildew resistant watermelon cultivars.