Location: Crop Improvement and Protection Research
Project Number: 2038-21530-003-012-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Nov 1, 2022
End Date: Apr 30, 2025
California’s Monterey County harvests 100,000 acres of lettuce (Lactuca sativa L.) annually at a value of $1.2 billion. In recent years, incidence of Impatiens Necrotic Spot Virus (INSV) has increased significantly causing devastating losses to lettuce production. Genetic resistance is the most economical and environmentally sound control method, but only partial resistance to INSV has been identified. Therefore, further characterization of this resistance and development of improved lettuce germplasm is imperative. A high incidence and symptom severity of INSV in trials at the USDA-ARS Spence Farm in Salinas provides a unique opportunity to evaluate germplasm under field conditions. A validated greenhouse protocol with manual inoculation and thrips virus transmission ensures the highest level of infection to confirm field results and ELISA and qPCR techniques allow precise quantification of virus in plant tissues. We will use these tools to meet the following objectives: 1) identify transgressive segregants from partially resistant crosses, 2) develop improved romaine germplasm, 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 Impatiens Necrotic Spot Virus (INSV) resistance. Three breeding populations have been chosen for evaluation and continued development: Population A combines INSV partial resistance from ‘Ancora’ with INSV partial resistance from ‘Amazona’; Population B introgresses INSV partial resistance from ‘Eruption’ into ‘Conquistador’ a romaine-type; and Population C introgresses INSV partial resistance from ‘Anais’ into ‘Salinas’ a crisphead-type. During the first year, germplasm from Populations A, B, and C will be evaluated under INSV in controlled, greenhouse conditions and under disease-free, field conditions. In the field, lines are evaluated for horticultural traits. In the greenhouse, seedlings are mechanically inoculated with INSV and then moved to a stable population of INSV-infected western flower thrips. Plants will be rated weekly for INSV (0-5, intensity scale of lesions and necrosis) and thrips (0-5, severity scale of damage) symptoms. During year two, selected lines from Populations A, B, and C will continue screening under INSV greenhouse experiments. All Populations will be evaluated for horticultural traits in disease-free, field conditions. Results of controlled and field experiments for Populations A, B, and C will be analyzed statistically and used to select lines with INSV resistance and desirable horticultural traits from within and between families. Selected lines will be evaluated in greenhouse and field trials to validate year one results and make best selections. Research results on lines with INSV 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 INSV resistance and lines with desirable horticultural traits will be distributed to public and private lettuce breeders to incorporate the traits in advanced breeding lines.