# Fine mapping, introgression, and KASP marker development for powdery mildew resistance in watermelon using an interspecific RIL population (Citrullus mucosospermus × C. lanatus)

## TL;DR

Researchers identified a gene region in watermelon that provides resistance to powdery mildew and developed markers to help breed resistant varieties.

## Contribution

A 54.8 kb QTL region on chromosome 2 was fine-mapped and validated KASP markers were developed for MAS of powdery mildew resistance in watermelon.

## Key findings

- A major QTL for powdery mildew resistance was fine-mapped to a 54.8 kb region on chromosome 2.
- Four KASP markers were developed and validated across multiple populations for marker-assisted selection.
- Resistance is controlled by a single dominant gene with a 3:1 segregation ratio in F2 populations.

## Abstract

A major QTL conferring powdery mildew resistance was fine mapped to a 54.8 kb region on chromosome 2 using an interspecific RIL population (Citrullus mucosospermus × Citrullus lanatus). Four co-segregating KASP markers were developed and validated across multiple populations, demonstrating their utility for marker-assisted selection.

Powdery mildew, caused by Podosphaera xanthii, is a major fungal disease that significantly affects watermelon production worldwide. Developing resistant cultivars through marker-assisted selection (MAS) offers an effective and sustainable strategy for disease management. In this study, a 54,772 bp quantitative trait locus (QTL) associated with powdery mildew resistance was mapped to chromosome 2 (30,111,475–30,166,247 bp) using an F11 recombinant inbred line (RIL) population derived from an interspecific cross between the resistant C. mucosospermus line USVL531-MDR and the susceptible C. lanatus line USVL677-PMS. Genetic analysis revealed that resistance is controlled by a single dominant gene, supported by a 3:1 segregation ratio observed in F2 populations. The mapped region harbored three lipoxygenase (LOX) genes and one 50S ribosomal protein L27-like gene. Four KASP markers were developed from SNPs located within four putative genes in the QTL region and were validated across multiple segregating populations, including the RIL (USVL531-MDR × USVL677-PMS) and two F2 populations (USVL531-MDR × ’Sugar Baby’ and PI 560003 × USVL677-PMS). These markers accurately differentiated resistant and susceptible individuals (R2 = 0.68–0.82) and exhibited 100% co-segregation with powdery mildew resistance in the RIL and two F2 populations, demonstrating their utility for MAS. The identified QTL and validated KASP markers will facilitate MAS for powdery mildew resistance breeding and enable future gene cloning work.

The online version contains supplementary material available at 10.1007/s00122-025-05079-4.