Breeding lettuce for resistance against Sclerotina minor

Authors: Simko, Ivan; SUBBARAO, KRISHNA - University Of California; Hayes, Ryan

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2023
Publication Date: 11/20/2023
Citation: Simko, I., Subbarao, K.V., Hayes, R.J. 2023. Breeding lettuce for resistance against Sclerotina minor. HortScience. 58(12):1526–1532. https://doi.org/10.21273/HORTSCI17399-23.
DOI: https://doi.org/10.21273/HORTSCI17399-23

Interpretive Summary: Lettuce drop is a persistent disease that causes economic loss in lettuce production around the world. In Coastal California, the largest lettuce producing region in the U.S., the disease is predominantly caused by the soil dwelling fungus Sclerotinia minor. The fungus infects plant crowns, roots, or senescing leaves. Infected plants display wilting, necrosis, collapse of the lettuce head and eventually death. Resistance to lettuce drop among commercial romaine cultivars is generally poor therefore an introgression of partial resistance from wild, primitive or heirloom accessions into modern cultivars could improve integrated management approaches to control the disease. Breeding methods for lettuce drop resistance are not well developed and hinder the development of new lettuce drop resistant cultivars. The objectives of this work were to develop a pedigree-based breeding method for introgression of lettuce drop resistance into modern romaine germplasm. Progeny from crosses between the partially resistant cultivar Eruption and the susceptible romaine cultivars Darkland and Hearts Delight were selected in a modified pedigree breeding scheme. Families were evaluated for disease incidence and selected for lettuce drop resistance in artificially infested field experiments conducted in the summer and fall. Infected plants of partially resistant lines commonly do not produce seed, and therefore selection of resistant plants from infested nurseries is not possible. Non-infested field experiments were used to selected individual plants possessing improved horticultural characters for seed production, but from within only resistant families. Evaluation and selection of progeny using this breeding scheme occurred from the F2:3 through the F5:6 generations. In all generations, superior resistance was identified in the Eruption × romaine crosses. The breeding scheme generated eight green colored romaine type inbred lines with better resistance than the romaine parent and better head weight than Eruption. Use of the new romaine lines as parents in backcrosses to romaine produced F2:3 families with high levels of resistance. The pedigree method used in this research can be implemented with any source of resistance but is constrained using family selection and the inability to directly select individual plants for resistance. Breeding schemes that employ single seed descent or molecular markers are alternative approaches that would enable selection for resistance on individual genotypes.

Technical Abstract: Lettuce drop caused by Sclerotinia minor is a damaging disease of romaine lettuce (Lactuca sativa L.) production in California. Introgression of partial resistance from wild, primitive or heirloom accessions into modern cultivars could improve integrated management approaches to control the disease. Breeding methods for lettuce drop resistance are not well developed and hinder the development of new lettuce drop resistant cultivars. The objectives of this work were to develop a pedigree-based breeding method for introgression of lettuce drop resistance into modern romaine germplasm. Progeny from crosses between the partially resistant cultivar Eruption and the susceptible romaine cultivars Darkland and Hearts Delight were selected in a modified pedigree breeding scheme. Families were evaluated for disease incidence and selected for lettuce drop resistance in artificially infested field experiments conducted in the summer and fall. Infected plants of partially resistant lines commonly do not produce seed, and therefore selection of resistant plants from infested nurseries is not possible. Non-infested field experiments were used to selected individual plants possessing improved horticultural characters for seed production, but from within only resistant families. Evaluation and selection of progeny using this breeding scheme occurred from the F2:3 through the F5:6 generations. In all generations, superior resistance was identified in the Eruption × romaine crosses. The breeding scheme generated eight green colored romaine type inbred lines with better resistance than the romaine parent and better head weight than Eruption. Use of the new romaine lines as parents in backcrosses to romaine produced F2:3 families with high levels of resistance. The pedigree method used in this research can be implemented with any source of resistance but is constrained by the use of family selection and the inability to directly select individual plants for resistance. Breeding schemes that employ single seed descent or molecular markers are alternative approaches that would enable selection for resistance on individual genotypes.