Watermelon Rootstock ‘Carolina Strongback’ Incorporates Disease and Pest Resistance from NPGS Germplasm

USDA-ARS, U.S. Vegetable Laboratory, 2700 Savannah Highway, Charleston, South Carolina 29414.

Clemson University, Coastal Research and Education Center, Charleston, South Carolina 29414.

Kansas State University, Department of Plant Pathology, Manhattan, Kansas 66506.

USDA-ARS, Mycology and Nematology Genetic Diversity and Biology Laboratory, 10300 Baltimore Ave, Beltsville, Maryland 20705.

Corresponding author: amnon.levi@usda.gov

OUTLINE

1. Summary
2. Problems addressed
3. Solutions developed
4. Impact
5. Germplasm
6. References and additional resources
7. Chapter information

1. SUMMARY

Fusarium wilt (FW) and root-knot nematodes (RKN) are the economically most significant soilborne diseases and pests of the U. S. watermelon crop. The citron watermelon (Citrullus amarus) is a valuable genetic resource for improving host-plant resistance to biotic and abiotic stresses in watermelon cultivars. Geneticists and plant pathologists at the USDA-ARS, U.S. Vegetable Laboratory in Charleston, South Carolina evaluated the NPGS citron watermelon germplasm collection and identified FW and RKN-resistant accessions. They further developed genomic tools (including DNA markers and a ‘genomic selection’-AI approach) to facilitate the incorporation of FW- and RKN-resistance genes into watermelon cultivars. In collaboration with a Clemson University extension agent, they developed the ‘Carolina Strongback’ rootstock for grafting to watermelon plants. Through a plant variety protection (PVP) agreement with Syngenta Seeds, Inc., approximately 31 million seeds of ‘Carolina Strongback’ were sold for watermelon grafting and planted on up to 22,000 acres worldwide between the years 2021 and 2025. Watermelon plants grafted with ‘Carolina Strongback’ rootstocks consistently produced high yields with no observed incidences of FW or RKN.

2. PROBLEMS ADDRESSED

Fusarium wilt (FW) and root-knot nematodes (RKN) are the most economically significant soilborne diseases and pests, of the U. S. watermelon crop. FW and RKN infestations can lead to yield losses of up to 100% and 30%, respectively. The total estimated damage caused by FW and RKN to American farmers can total as much as $180 million annually, which is about 25% of the total yearly farm gate value of watermelon in the USA.

3. SOLUTIONS DEVELOPED

Geneticists and plant pathologists at the USDA-ARS, U.S. Vegetable Laboratory in Charleston, South Carolina evaluated the NPGS citron watermelon collection and identified accessions resistant to FW and RKN. They further identified genes associated with resistance to these pathogens and pests and developed genomic tools (including DNA markers and a ‘genomic selection’-AI approach) to facilitate the incorporation of resistance genes into watermelon cultivars. They developed a citron rootstock named ‘Carolina Strongback’ that proved a highly valuable rootstock for grafting watermelon plants by preventing FW and RKN infestations while producing high quality seedless watermelons in fields around the world.

4. IMPACT

1) Citrullus amarus germplasm lines were developed from NPGS germplasm, released to the public, and have been requested by most seed companies for improving FW and RKN host-plant resistance in elite watermelon cultivars.

2) DNA markers closely linked to FW race 2 resistance are being applied with ‘genomic selection’ methodology by seed company breeders so as to incorporate FW and/RKN resistance into their elite watermelon lines.

3) The ‘Carolina Strongback’ rootstock used for grafting watermelon plants proved highly effective in preventing FW and RKN infestations. Through a plant variety protection (PVP) agreement with Syngenta Seeds, Inc., approximately 31 million seeds of ‘Carolina Strongback’ were sold for watermelon grafting and planted as many as 22,000 acres worldwide (rough estimate) between the years 2021 and 2025. Watermelon plants grafted with ‘Carolina Strongback’ consistently produced high yields with no observed incidences of FW or RKN.

5. GERMPLASM

NPGS genebank accessions, crop wild relatives, and breeding lines of Citrullus spp. and Citrullus amarus were evaluated for FW and RKN resistance. Host-plant resistance was identified in several accessions, including: PI 482246, PI 482252, USVL246-FR2, USVL252-FR2, ‘Carolina Strongback’.

6. REFERENCES AND ADDITIONAL RESOURCES

Biswas A, Regmi H, Waldo BD, Branham S, Wechter P, Kousik S, Jarquin D, Levi A, Rutter WB. 2026. Genomic selection of root-knot nematode (Meloidogyne enterolobii) resistance in watermelon wild relatives (Citrullus amarus). The Plant Genome 19:e70203. https://doi.org/10.1002/tpg2.70203

Biswas A, Wechter P, Ganaparthi V, Jarquin D, Kousik S, Branham S, Levi A. 2025. Comparative genomic prediction of resistance to Fusarium wilt (Fusarium oxysporum f. sp. niveum race 2) in watermelon: Parametric and nonparametric approaches. Theoretical and Applied Genetics 138:35–11. https://doi.org/10.1007/s00122-024-04813-8

Branham SE, Levi A, Farnham MW, Wechter WP. 2017. A GBS-SNP-based linkage map and quantitative trait loci (QTL) associated with resistance to Fusarium oxysporum f. sp. niveum race 2 identified in Citrullus lanatus var. citroides. Theoretical and Applied Genetics 130:319–330. https://doi.org/10.1007/s00122-016-2813-0

Branham SE, Wechter WP, Ling KS, Chanda B, Massey L, Zhao G, Guner N, Bello M, Kabelka E, Fei Z, Levi A. 2020. QTL mapping of resistance to Fusarium oxysporum f. sp. niveum race 2 and Papaya ringspot virus in Citrullus amarus. Theoretical and Applied Genetics 133:677–687. https://doi.org/10.1007/s00122-019-03500-3

Ganaparthi V, Wechter W, Levi A, Branham S. 2024. Mapping and validation of Fusarium wilt race 2 resistance QTL from Citrullus amarus line USVL246-FR2. Theoretical and Applied Genetics 137:91. https://doi.org/10.1007/s00122-024-04595-z

Ganaparthi VR, Rennberger G, Wechter WP, Levi A, Branham SE. 2023. Genome-wide association mapping and genomic prediction of Fusarium wilt race 2 resistance in the USDA Citrullus amarus collection. Plant Disease 107:3836–3842. https://doi.org/10.1094/PDIS-02-23-0400-RE

Levi A, Jarret R, Kousik S, Wechter WP, Nimmakayala P, Reddy UK. 2017. Genetic resources of watermelon, p. 99–122. In: Grumet R, Katzir N, Garcia-Mas J (eds). Genetics and genomics of Cucurbitaceae. Springer International Publishing, Cham, Switzerland. https://doi.org/10.1007/7397_2016_34

Levi A, Thies JA, Wechter PW, Farnham M, Weng Y, Hassell R. 2014. USVL-360, a novel watermelon tetraploid germplasm line. HortScience 49:354–357. https://doi.org/10.21273/HORTSCI.49.3.354

Levi A, Thies JA, Wechter WP, Harrison HF, Simmons AM, Reddy UK, Nimmakayala P, Fei Z. 2013. High frequency oligonucleotides: targeting active gene (HFO-TAG) markers revealed wide genetic diversity among Citrullus spp. accessions useful for enhancing disease or pest resistance in watermelon cultivars. Genet Resources an Crop Evolution 60:427–440. https://doi.org/10.1007/s10722-012-9845-3

Thies JA, Ariss JJ, Hassell RL, Buckner S, Levi A. 2014a. Accessions of Citrullus lanatus var. citroides are valuable rootstocks for grafted watermelon in fields infested with root-knot nematodes. HortScience 50:4–8. https://doi.org/10.21273/HORTSCI.50.1.4

Thies JA, Levi A, Ariss JJ, Hassell RL. 2014b. RKVL-318, a root-knot nematode-resistant watermelon line as rootstock for grafted watermelon. HortScience 50:141–142. https://doi.org/10.21273/HORTSCI.50.1.141

Waldo B, Branham S, Levi A, Wechter W, Rutter W. 2022. Distinct genomic loci underlie quantitative resistance to Meloidogyne enterolobii galling and reproduction in Citrullus amarus. Plant Disease 107:2126–2132. https://doi.org/10.1094/PDIS-09-22-2228-RE

Wechter WP, Kousik C, McMillan M, Levi A. 2012. Identification of resistance to Fusarium oxysporum f. sp. niveum race 2 in Citrullus lanatus var. citroides plant introductions. HortScience 47:334–338. https://journals.ashs.org/view/journals/hortsci/47/3/article-p334.xml

7. CHAPTER INFORMATION

Citation: Levi A, Wechter P, Branham SE, Rutter W, Waldo B, Kousik S, Thies J, Regmi H, Katawczik M, Massey L, Ikerd J, Ganaparthi V, Biswas A. 2026.

Watermelon Rootstock ‘Carolina Strongback’ Incorporates Disease and Pest Resistance from NPGS Germplasm. In: Volk GM, Chen KY, Byrne PF, Bretting PK (Eds.) Plant Genetic Resources: Success Stories. Fort Collins, Colorado: Colorado State University. Date accessed. Available from https://colostate.pressbooks.pub/pgrsuccessstories/chapter/watermelon-carolina-strongback/

Content originally submitted: March 18, 2026

Date of publication: July 28, 2026

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