Location: Crop Improvement and Protection Research
2018 Annual Report
Objectives
The focus of this research program is on quality traits, resistances to diseases, insects and abiotic stresses of lettuce, spinach and melon considered by the respective industries and the scientific community to be the most critical to production. We will develop elite germplasm and cultivars with improved quality and productivity, and new knowledge of the genetics and breeding of lettuce, spinach, and melon. Specifically, during the next five years we will focus on the following objectives.
Objective 1: Discover and understand novel sources of resistance in lettuce to priority diseases and insects, tolerance to unfavorable abiotic factors (including physiological defects), and improved phytonutrient content; discover trait-linked molecular markers, and use these resources to develop and release improved lettuce germplasm and/or finished varieties.
• Subobjective 1A: Corky Root
• Subobjective 1B: Downy Mildew
• Subobjective 1C: Fusarium Wilt
• Subobjective 1D: Leafminer
• Subobjective 1E: Lettuce Drop
• Subobjective 1F: Phytonutrients
• Subobjective 1G: Postharvest Quality
• Subobjective 1H: Tipburn
• Subobjective 1I: Impatiens necrotic spot virus
• Subobjective 1J: Verticillium Wilt
Objective 2: Discover and understand novel sources of resistance in spinach to new and emerging diseases (especially downy mildew) and insects (including leaf miner), and develop and release improved spinach germplasm and/or finished varieties.
• Subobjective 2A: Spinach Downy Mildew
• Subobjective 2B: Leafminer
• Subobjective 2C: Linuron Herbicide Tolerance
Objective 3: Discover and understand novel sources of resistance in melon to priority diseases and insect pests, and develop and release improved cantaloupe and honeydew germplasm and/or finished varieties with durable resistance.
• Subobjective 3A: Resistance to Powdery Mildew
• Subobjective 3B1: Resistance to Sweetpotato Whitefly
• Subobjective 3B2: Determine inheritance of antixenosis
• Subobjective 3B3: Introgression of Antixenosis
Approach
1A: Corky Root. Approach: Combine resistances to corky root, leafminer, downy mildew, lettuce mosaic virus, & tipburn, & nutritional traits; pedigree selection & backcross for type.
1B: Downy Mildew. Approach: Map QTL in 2 F6 RIL populations & develop breeding lines with improved level of resistance. Cross resistant RIL & accessions; pedigree selection & backcross for type.
1C: Fusarium Wilt. Approach: Develop Fusarium wilt-resistance for the Salinas Valley by crossing advanced resistant desert selections with ‘Salinas’; backcross resistant F2 selection to ‘Salinas’, repeat to BC4F4.
1D: Leafminer Approach: Introgress leafminer resistance to different lettuce types by intercrossing resistance sources, then crossing them with breeding lines for combined resistances. Pedigree selection to F6.
1E: Lettuce Drop. Approach: Map QTL for resistance in a F6 RIL population; develop romaine lettuce with improved resistance using most resistant RIL & other accessions. Pedigree selection & backcross for type.
1F: Phytonutrients. Approach: Improve phytonutrient content of lettuce by crossing high carotenoid, anthocyanin, and antioxidant content sources with elite cultivars. Pedigree selection & backcross for type.
1G: Postharvest Quality. Approach: Develop tools to improve lettuce shelf life by combining automatic phenotyping, mapping & molecular markers for MAS; release breeding lines with extended shelf life.
1H: Tipburn. Approach: Develop romaine breeding lines with reduced incidence of tipburn using pedigree selection and backcrossing of advanced lines; select in desert and coastal environments.
1I: Impatiens necrotic spot virus. Approach: Identify resistance sources in Salinas & Pullman accessions in greenhouse tests; mechanical and thrips inoculations. Cross most resistant with elite cultivars.
1J: Verticillium Wilt. Approach: Identify higher levels of resistance to V. dahliae race 2 in Salinas & Pullman lettuce collection. Cross most resistant accessions with elite cultivars.
2A: Spinach Downy Mildew. Approach: Open-pollinated (OP) seed from resistant hybrid spinach cultivars will be OP with susceptible ‘Viroflay’; recurrent selection to combine resistances in OP lines.
2B: Leafminer. Approach: Breed for leafminer resistance against both stings and mines using recurrent selection starting with highest sources of resistance.
2C: Linuron Herbicide Tolerance. Approach: Recurrent selection to increase tolerance to Linuron in field tests.
3A: Resistance to Powdery Mildew. Approach: Introgress resistance in PI 313970 to races 1, 2, 3.5, 5, and S using F2 and F2:3 selections in greenhouse & field tests. Pedigree selection & backcross for type.
3B1: Resistance to Sweetpotato Whitefly. Approach: Compare antixenosis in 4 accessions using individual & group responses, odor-based assays, electrical penetration graphs, & candidate compounds.
3B2: Determine inheritance of antixenosis. Approach: Determine whether antixenosis in PI 122847 is simply inherited or quantitative using Y-tube assays of F2.
3B3: Introgression of Antixenosis. Approach: Introgress antixenosis in PI 122847 to elite western shipping type melon using backcrossing and inbreeding.
Progress Report
This report documents progress for project 2038-21530-002-00D, which started in April 2018 and continues research from project 2038-21530-001-00D, "Genetic Enhancement of lettuce, Spinach, Melon, and Related Species."
In support of Sub-objectives 1A, 1D, and 1F, scientists continued to self, cross, and select resistances to corky root and leafminers, and phytonutrient improvement, respectively, along with horticultural traits. Breeding lines in advanced generations were tested in field trials with control varieties and commercial cultivars.
Related to Sub-objective 1G, there is high demand for lettuce cultivars with slow decay after fresh-cut processing. The processing industry also needs approaches for automated, non-destructive evaluation of lettuce decay in modified atmosphere packaging. Development of a method for automatic detection of lettuce decay is, therefore, of great importance to the industry. High throughput, non-destructive, phenotyping could be used by the lettuce processing industry to quantitatively and objectively assess decay of fresh-cut lettuce, and by lettuce breeding companies to identify cultivars with improved shelf life. Scientists purchased and installed FluorCam, an instrument that allows for automated analysis of chlorophyll fluorescence. Plants from 500 lettuce accessions that were developed through single-seed descent approach were sampled for DNA analyses and increased for future field experiments. This plant material will be used for genome-wide association mapping of genes related to post-harvest quality of lettuce and its stability across environments.
Sub-objective 1H is to develop romaine breeding lines with reduced incidence of tipburn. In May 2017, 217 breeding lines were planted in Salinas field experiments to evaluate resistance to tipburn. Ten F2 populations segregating for tipburn resistance were evaluated in replicated field plots in 2018. Selected plants will be advanced in the breeding program.
Sub-objective 1I is to identify lettuce germplasm with resistance to Impatiens necrotic spot virus (INSV). The thrips colony (INSV vector) is under development in preparation for germplasm screening.
Sub-objective 1J is to identify higher levels of resistance to Verticillium dahliae race 2 and develop resistant iceberg lettuce. Preliminary experiments are underway to compare results from greenhouse and growth chamber tests to identify the optimal growing conditions for Verticillium wilt.
Under Sub-objectives 2A, 2B, and 2C, scientists continued to make crosses and selections for resistances to downy mildew and leafminer, and tolerance to Linuron herbicide, respectively, as well as horticultural traits by using a recurrent selection method. Selected plants were transplanted into isolators to produce seeds for further rounds of selection. ARS researchers also continued grant-funded spinach genomic studies with collaborators at University of Arkansas to find molecular markers for disease resistance and horticultural traits. In another grant-funded research, a postdoc is continuing the investigation of the roles of oospores and biofungicide in downy mildew development in collaboration with a pathologist.
Sub-objective 3A relates to powdery mildew resistance in PI 313970. More than 100 F2:F3 Top Mark x PI 313970 progenies were produced in field and greenhouse plantings in the Imperial Valley and Salinas, California. This material will also be used for mapping resistance to Cucurbit yellow stunting disorder virus. In a related study, more than 100 F2:F3 PI 313970 x PI 482420 (TGR 155) progenies were produced in a greenhouse.
Sub-objective 3B relates to sweetpotato whitefly resistance in melon. A non-assistance cooperative agreement (NACA) was established with the University of California, Riverside for laboratory studies of sweetpotato whitefly-melon interactions.
Accomplishments
