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United States Department of Agriculture

Agricultural Research Service

Research Project: Evaluation, Enhancement, Genetics and Breeding of Lettuce, Spinach, and Melon

Location: Crop Improvement and Protection Research

2008 Annual Report

1a. Objectives (from AD-416)
The focus of this research program is on quality traits, diseases, and insect pests of lettuce, spinach and melon considered by the respective industries and the scientific community to be the most critical to production. The overall goal is the development of elite germplasm and cultivars with improved quality and productivity, and new knowledge of the genetics and breeding of lettuce, melon and spinach. Genetic improvement of lettuce, melon, and spinach. Identify genetic variation controlling key horticultural traits, and determine their genetic bases, and develop and release elite germplasm and cultivars with improved quality and productivity.

1b. Approach (from AD-416)
Collect, identify, characterize, and evaluate wild and unadapted germplasm of lettuce, spinach, and melon. Evaluate germplasm for resistance to virus (lettuce mosaic, lettuce dieback, big vein, cucurbit yellow stunting disorder virus) fungal (downy mildew, Fusarium wilt, sclerotinia, powdery mildew, Verticillium wilt) bacterial (Stemphylium Leafspot) and insect (lettuce aphid, leafminer). Improve quality including nutritional content, shelf life, and reduced oxalic acid. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine inheritance and linkage relationships of phenotypic, biochemical and molecular markers. Devise techniques for evaluating insect-host interactions and selecting for resistance to insects in field and greenhouse tests. Replacing 5305-21000-011-00D (04/08).

3. Progress Report
For lettuce, our objectives are to incorporate resistance to several diseases, insects, and physiological defects into iceberg, romaine, and mixed lettuce cultivars and breeding lines adapted for coastal and desert production. Greenhouse and field experiments were conducted in Coastal California, the Imperial Valley of CA, and the Yuma Valley of AZ to develop lettuce resistant to lettuce big vein disease, lettuce drop/Sclerotinia species, Verticillium wilt, Fusarium root rot, lettuce dieback/tombusviruses, corky root, leafminer, and lettuce aphid. Breeding for increased nutritional content was also conducted. We confirmed resistance in previously identified germplasm to Verticillium wilt, Fusarium wilt and lettuce aphid. New candidate sources of resistance were identified to race 2 isolates of Verticillium dahliae and for extended shelf life in modified atmosphere packaging. Selections were taken from breeding populations, and advanced breeding lines were evaluated, as part of breeding for resistance to big vein disease, lettuce drop, Verticillium wilt, dieback, corky root and for increased nutritional content. Genetic studies concurrent with breeding programs are being conducted to determine the inheritance of resistance to big vein disease, lettuce drop, leafminers, and Verticillium wilt. We are working to develop molecular markers linked to genes conferring resistance to verticillium wilt, leafminer, and dieback. Melon research continued to emphasize host plant resistance to powdery mildew and Cucurbit yellow stunting disorder virus (CYSDV) in replicated field trials in Imperial Valley, CA and Yuma, AZ, and in greenhouses tests with University of California collaborator Mike Coffey. Powdery mildew research emphasized identification of additional new sources of resistance to race S. In a September 2008 planting in Imperial Valley, CYSDV research will focus on inheritance of resistance in PI 313970, the relationship between resistance to Lettuce infectious yellows virus resistance and CYSDV in PI 313970, variation for expression of confirmed resistance in TGR-1551 and PI 313970, and evaluation of 100 melon accessions for potential new sources of resistance to CYSDV. Further progress was made in selection for elite, monoecious, western U.S. shipping type melon germplasm. Spinach genotypes with resistance to Stemphylium leafspot or low oxalic acid content were confirmed. They are being used in our breeding program to increase the level of resistance or further decrease the oxalate content in spinach. Breeding for leafminer resistance continues. With funding from the California Leafy Greens Research Program, we are starting new projects to screen spinach germplasm for resistance to downy mildew and Verticillium wilt. This research addresses National Program 301, Component 3.

4. Accomplishments
1. Identified genetic variation in cultivated lettuce for shelf-life of minimally processed salad in modified atmosphere packaging. Pre-packaged salad is an important market for lettuce produces, its shelf-life is extended using modified atmosphere packaging (MAP). While industry reports that not all modern cultivars perform well in this packaging system, nothing is known about the genetics and breeding of lettuce for shelf-life in these packages. We develop two high throughput methods to evaluate shelf-life in (MAP), and determined the lettuce has genetic variation for extended shelf-life. Several important sources of disease resistance had unacceptably short shelf-life, and highlights the need to develop breeding methods to select against this negative trait. This accomplishment aligns with National Program 301 component 3, and address problem statement 3C.

2. Resistance to Cucurbit Yellow Stunting Disorder Virus in Melon. Evaluated germplasm for resistance to this virus that appeared in Arizona and California for the first time in Fall 2006. Confirmed high level but not immune resistance to CYSDV in PI 313970 and found it comparable to resistance in TGR 1551. An evaluation of 96 accessions from India revealed three putative resistant plants. CYSDV is a devastating virus disease of melon that has been known in the Middle East, North Africa and southern Europe since about 1990. The research was led by scientists from the U.S. Agricultural Research Station, Salinas, CA in collaboration with scientists from the University of Arizona and University of California, Davis. This accomplishment aligns with National Program 301, Genomic Characterization and Genetic Improvement and addresses problem statement 3B.

3. Microsatellite-enriched genomic libraries for lettuce. Microsatellites are highly informative markers suitable for the creation of framework maps. Furthermore, their co-dominant nature makes them ideal for use in marker assisted selection, but previously developed microsatellites for lettuce are not publicly available. We have, therefore, created three microsatellite-enriched genomic libraries for public use. We identified 97 unique sequences and designed 109 primer pairs from the sequencing of 192 clones. To date, 40 new loci have been identified on the L. sativa (Salinas) x L. serriola (UC96US23) progeny, provided by R. Michelmore (UC Davis) and more are in process. Later, the microsatellite primers will be analyzed for their level of polymorphism on a set of 32 cultivars representing different horticultural types of lettuce. The long-term goal for these SSR markers is to be used in mapping and marker assisted selection of traits agronomically relevant, including quantitative traits. This accomplishment aligns with National Program 301, Genomic Characterization and Genetic Improvement and addresses problem statement 3B.

5. Significant Activities that Support Special Target Populations
None to report.

Review Publications
Mou, B. 2008. Lettuce. Handbook of Plant Breeding, Vol. I, Vegetables I, Asteraceae, Brassicaceae, Chenopodicaceae, and Cucurbitaceae, J. Prohens and F. Nuez, ed., Springer, New York, p. 75-116.

Hu, J., Mou, B., Vick, B.A. 2007. Genetic diversity of 38 spinach (Spinacia oleracea L.) germplasm accessions and 10 commercial hybrids assessed by TRAP markers. Genetic Resources and Crop Evolution. 54:1667-1674.

Simko, I., Van Den Berg, J.H., Vreugdenhil, D., Ewing, E.E. 2008. Mapping loci for chlorosis associated with chlorophyll b deficiency in potato. Euphytica Vol 162, Pages 99-107, 2008.

Mou, B. 2007. Leafminer-resistant spinach germplasm 03-04-63. HortScience. Vol. 42(7):1717-1718.

Last Modified: 08/16/2017
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