New genetic tools help breed disease-resistant watermelons

Jim Crocker
11th November, 2025

New genetic tools help breed disease-resistant watermelons

This figure from the study shows contrasting powdery mildew resistance in watermelon genotypes and highlights the desirable fruit characteristics—red flesh, firm texture, and non-bitter taste—selected for in resistant recombinant inbred lines derived from crosses between susceptible and resistant Citrullus species.

Image adapted from: Kumar et al. / CC BY (Source)

Key Findings

  • Researchers identified a specific region on watermelon chromosome 2 strongly linked to resistance against powdery mildew, a common fungal disease
  • This resistance is governed by a single dominant gene, meaning inheriting one copy provides protection, and is pinpointed to a 54.8 kb region of the watermelon genome
  • Four DNA markers (KASP markers) were developed from this region and accurately predict disease resistance in different watermelon varieties, aiding faster breeding of resistant plants
Powdery mildew is a significant problem for watermelon growers, caused by the fungus Podosphaera xanthii. This disease can severely reduce crop yields, making the development of resistant watermelon varieties crucial for sustainable agriculture. Researchers at the USDA ARS Vegetable Laboratory have recently identified a specific region of the watermelon genome strongly linked to resistance against this disease[1]. The study focused on pinpointing the genetic location responsible for powdery mildew resistance. This was achieved by creating a population of recombinant inbred lines (RILs). RILs are essentially genetically uniform plants created by repeatedly crossing two very different parent plants – in this case, a wild watermelon species (Citrullus mucosospermus) known for its resistance and a cultivated watermelon (Citrullus lanatus) that is susceptible to the disease. By analyzing where the resistance trait appeared in these RILs, scientists could narrow down the region of the genome responsible. This process, known as quantitative trait locus (QTL) mapping, relies on identifying sections of DNA that consistently co-segregate with the desired trait – in this case, resistance to powdery mildew. The researchers used a sophisticated software package, R/qtl2, to analyze the genetic data and identify the QTL[2]. Their work revealed a major QTL conferring resistance located on chromosome 2, spanning a relatively small 54.8 kilobase (kb) region of the watermelon genome. A kilobase is a unit of measurement equal to 1,000 base pairs of DNA. Further genetic analysis indicated that resistance is governed by a single, dominant gene. This means that if a plant inherits just one copy of the resistance gene, it will exhibit resistance to powdery mildew. This finding builds on earlier work which showed that resistance to powdery mildew in watermelon can be controlled by different numbers of genes depending on the plant’s developmental stage[3]. Specifically, resistance at the early cotyledon stage is controlled by a single gene, while resistance at later stages requires the interaction of three genes. The current study focuses on identifying the single dominant gene responsible for resistance, providing a more focused approach to breeding. Within the identified 54.8 kb region, the researchers found several candidate genes, including three lipoxygenase (LOX) genes and one gene similar to a ribosomal protein L27 gene. To facilitate the use of this discovery in breeding programs, the team developed four KASP (Key Amplification-Specific Polymerase) markers. These markers are essentially DNA tags that can be used to quickly and accurately identify plants carrying the resistance gene. They function by detecting specific variations in the DNA sequence within the QTL region. The KASP markers were then tested across multiple watermelon populations, including the original RIL population and two additional F2 populations (created by crossing resistant and susceptible parents). The markers consistently and accurately distinguished between resistant and susceptible plants, with a high degree of accuracy (R2 values of 0.68-0.82). This demonstrates their potential for marker-assisted selection (MAS). MAS allows breeders to select for resistance without having to grow out plants and expose them to the disease, significantly speeding up the breeding process. Interestingly, the ribosomal protein L27 gene identified within the QTL region has connections to plant immunity. Research has shown that similar proteins in other organisms, like amphioxus, possess antimicrobial properties[4]. These proteins, derived from ribosomes, can disrupt bacterial cell membranes and boost the immune response. While this study doesn’t directly demonstrate antimicrobial activity of the watermelon ribosomal protein L27, it raises the possibility that the resistance mechanism involves a similar immune-boosting function. The identification of this major QTL and the development of these reliable KASP markers represent a significant step forward in breeding powdery mildew-resistant watermelon varieties. These tools will enable breeders to efficiently incorporate resistance into commercially important cultivars, reducing the reliance on chemical controls and promoting sustainable watermelon production.

AgricultureGeneticsPlant Science

References

Main Study

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

Published 8th November, 2025

https://doi.org/10.1007/s00122-025-05079-4

Related Studies

2) R/qtl2: Software for Mapping Quantitative Trait Loci with High-Dimensional Data and Multiparent Populations.

https://doi.org/10.1534/genetics.118.301595

3) Inheritance of Resistance to Powdery Mildew Race 1W in Watermelon.

https://doi.org/10.1094/PHYTO-02-15-0048-R

4) Identification of a novel antimicrobial peptide from amphioxus ribosomal protein L27.

https://doi.org/10.1016/j.fsi.2024.110063


Related Articles

An unhandled error has occurred. Reload 🗙