Location: Crop Improvement and Protection Research
Project Number: 2038-21530-002-028-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Nov 1, 2019
End Date: Apr 30, 2022
Verticillium dahliae is highly destructive to lettuce production in California and total losses of crisphead-types are typical. Genetic resistance is the most economical and environmentally sound control method. Complete resistance to V. dahliae Race 1 is conferred by the single, dominant gene Vr1, but only partial resistance has been identified for Race 2. Vr1 is currently effective in many commercial fields, but V. dahliae Race 2 isolates are being introduced on spinach seed, a common rotation crop with lettuce. Therefore, characterization of resistance to Race 2 and development of improved lettuce germplasm is imperative. Our objectives are to 1) pyramid Vr1 and resistance to Race 2 in a single genotype, 2) identify transgressive segregants from partially-resistant crosses, and 3) develop improved crisphead germplasm. Success will be indicated by releasing new lettuce germplasm, publications, citations, and requests for seeds and information.
A modified pedigree breeding method will be used to select for V. dahliae resistance. Three breeding populations have been chosen for evaluation and continued development; Population A combines Vr1 resistance in a crisphead-type with Race 2 partial resistance from PI 171674, Population B combines Race 2 partial resistance from 11-G99 with Race 2 partial resistance from PI 171674, and Population C introgresses Race 2 partial resistance from PI 171674 into ‘Salinas’ a crisphead-type. During the first year, germplasm from Populations A, B, and C will be evaluated for reaction to V. dahliae Race 2 in controlled, growth room conditions and a disease-free, field test. In the field, lines are evaluated for horticultural traits. In the greenhouse, seed is planted in microsclerotia-infested soil, followed by conidia inoculations at two, three, and four weeks after sowing. Plants will be rated for foliar (0-5, intensity scale of foliar wilting, chlorosis, and necrosis) and vascular ( 0-5, severity scale of taproot vascular discoloration) symptoms when plants bolt or show first flower. During year two, selected lines from Populations A, B, and C will continue screening for reaction to Race 2 in controlled experiments, and lines from Population A will be tested for reaction to V. dahliae Race 1 in a field test. All Populations will be evaluated for horticultural traits in a disease-free, field test. Results of controlled and field experiments for Populations A, B, and C will be analyzed statistically and used to select lines with V. dahliae resistance and desirable horticultural traits from within and between families. Selected lines will be evaluated in controlled and field trials to validate year one results and make best selections. Research results on lines with V. dahliae resistance and desirable horticultural traits will be presented in annual professional and industry meetings and published in peer-reviewed journals. Feedback from the audience will be used to improve future research. Seed of lines with V. dahliae resistance and lines with desirable horticultural traits will be distributed to public and private lettuce breeders to incorporate the traits in advanced breeding lines.