Vegetables and Pulses
NPGS Germplasm Provides U.S. Lettuce Crop with Disease Resistance
GERMPLASM OF LETTUCE WILD RELATIVES PROVIDES LETTUCE WITH MULTI-RACE RESISTANCE TO BACTERIAL LEAF SPOT
Ivan Simko
USDA-ARS, Sam Farr United States Crop Improvement and Protection Research Center, Salinas, California 93905.
Corresponding author: ivan.simko@usda.gov
OUTLINE
- Summary
- Problems addressed
- Solutions developed
- Impact
- Germplasm
- Additional resources
- Chapter information
1. SUMMARY
USDA-ARS researchers identified critical sources of resistance to multiple races of Bacterial Leaf Spot (BLS) by evaluating nearly 500 lettuce germplasm accessions, including several crop wild relatives (CWR). These genetic resources, primarily from the lettuce CWR Lactuca serriola, are now contributing to the development of hardy, disease-resistant cultivars for the lettuce industry.
The goal of this project was to identify novel genetic sources of resistance to multiple races of BLS within Lactuca germplasm accessions and to characterize the genetic basis of resistance.
2. PROBLEMS ADDRESSED
BLS, caused by the bacterium Xanthomonas hortorum pv. vitians, is a devastating disease for the lettuce industry, particularly in regions with high humidity or during overhead irrigation. In high-density plantings like those for ‘spring mix’ or ‘baby leaf’ lettuce, an outbreak can render an entire crop unmarketable, leading to total economic loss. The emergence of at least three distinct races of the pathogen has made existing resistance in commercial cultivars insufficient, creating an urgent need for new genetic defenses.
3. SOLUTIONS DEVELOPED
Scientists evaluated approximately 500 Lactuca germplasm accessions from the NPGS Pullman genebank and associated germplasm collections to find new genes for disease resistance. The team discovered that, although host-plant resistance in cultivated lettuce was limited, crop wild relatives—specifically L. serriola—held the key. They identified several NPGS accessions, including PI 491114 and PI 491108, that provide a hypersensitive defense response to Race 2. Most important, the accession ARM09-161 (deposited in the NPGS as W6 37134) demonstrated robust broad-spectrum resistance, offering defense traits that breeders can now incorporate into commercial lettuce types.
Collaborators involved in developing solution:
- Ivan Simko, Ryan Hayes, Jinita Sthapit Kandel, Wei Zhou, Beiquan Mou, Mark Trent: USDA-ARS, Salinas, California, USA
- German Sandoya, Lis Rodrigues-Porto, William Wadlington, Amanda Carroll: University of Florida, Belle Glade, Florida, USA
- Ales Lebeda: Palacky University, Olomouc, Czech Republic
- Carolee Bull, Emma Rosenthal: Pennsylvania State University, State College, Pennsylvania, USA
- Quentin Read: USDA-ARS, Raleigh, North Carolina, USA
- Richard Michelmore, Maria Jose Truco: University of California, Davis, California, USA
- Brigitte Maisonneuve: Institut National de la Recherche Agronomique (INRA), Avignon, France
4. IMPACT
By identifying and mapping these host-plant resistance genes—including specific genes for protecting baby leaf lettuce—this research provides breeders with the genetic tools and germplasm needed to protect the U.S. lettuce supply. This ensures that growers can maintain high-quality, disease-free production even in the face of evolving pathogen races, reducing the risk of total lettuce crop failure and minimizing the need for chemical applications.
5. GERMPLASM
More than 500 NPGS accessions of cultivated lettuce (L. sativa) and its related crop wild relative species (L. serriola, L. saligna, L. aculeata, L. altaica, L. virosa, L. perennis) were evaluated for host-plant resistance to bacterial leaf spot disease. The accessions PI 491114 and PI 491108 provided a hypersensitive defense response to Race 2 of BLS, while the accession ARM09-161 demonstrated broad-spectrum resistance to the disease.
6. ADDITIONAL RESOURCES
Sandoya GV, Maisonneuve B, Truco MJ, Bull CT, Simko I, Trent M, Hayes RJ, Michelmore RW. 2019. Genetic analysis of resistance to bacterial leaf spot in the heirloom lettuce cultivar Reine des Glaces. Molecular Breeding. https://doi.org/10.1007/s11032-019-1072-6
Sandoya GV, Rosenthal E, Simko I, Rodrigues-Porto LN, Wadlington WH, Bull CT, Carroll A. 2022. Lettuce (Lactuca sativa L.) germplasm resistant to bacterial leaf spot caused by race 1 of Xanthomonas hortorum pv. vitians. Journal of Plant Pathology. https://doi.org/10.1007/s42161-022-01123-0
Sandoya GV, Trent M, Hayes RJ, Lebeda A, Rosenthal E, Simko I, Bull CT. 2025. Differential sources of resistance from Lactuca serriola against three races of Xanthomonas hortorum pv. vitians causing bacterial leaf spot of lettuce. Plant Disease. https://doi.org/10.1094/PDIS-06-24-1239-RE
Sthapit Kandel J, Sandoya GV, Zhou W, Read QD, Mou B, Simko I. 2022. Identification of quantitative trait loci associated with bacterial leaf spot resistance in baby leaf lettuce. Plant Disease. https://doi.org/10.1094/PDIS-09-21-2087-RE
8. CHAPTER INFORMATION
Citation: Simko I. 2026. NPGS Germplasm Provides U.S. Lettuce Crop with Disease Resistance. 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/lettuce-npgs-germplasm-bls-resistance/
Content originally submitted: April 21, 2026
Date of publication: July 20, 2026
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