Breeding Lettuce for Resistance against Sclerotinia minor

Lettuce drop caused by Sclerotinia minor is a damaging disease of romaine let-tuce (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 the disease. Breeding methods for lettuce drop resistance are not well developed and hinder the development of new lettuce drop-resistant cultivars. The objective of this work was 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 in -fested field experiments conducted in the summer and fall. Infected plants of partially resis-tant lines commonly do not produce seed, and therefore selection of resistant plants from infested nurseries is not possible. Noninfested field experiments were used to select individ-ual plants with improved horticultural characteristics for seed production, but from within resistant families only. Evaluation and selection of progeny using this breeding scheme oc-curred from the F2:3 through the F5:6 generations. In all generations, superior resistance was identified in the 'Eruption' x romaine crosses. The breeding scheme generated eight green 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 back-crosses 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 select individual plants for resistance directly. Breeding schemes that use single seed descent or molecular markers are alternative approaches that would enable selection for resistance on indi-vidual genotypes.

Automated summary

This study developed a pedigree-based breeding method to introgress lettuce drop resistance into modern romaine germplasm. Through evaluation and selection of progeny, eight green romaine-type inbred lines with better resistance and head weight were generated. However, the breeding scheme used in this research has limitations in individual plant selection for resistance.