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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

2010 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
This project addresses breeding and genetics of lettuce, spinach and melon. Our objectives are to incorporate resistance to several diseases, insects, and physiological defects. Genetic studies are being conducted to determine the inheritance of resistance all traits of interest. Major efforts targeted resistance to lettuce big vein disease, lettuce drop (incited by two Sclerotinia species), Verticillium wilt, Fusarium wilt, lettuce dieback (incited by tombusviruses), bacterial leaf spot, corky root, leafminer, lettuce aphid and tipburn, and multiple disease resistance. Minor programs addressed resistance to powdery mildew and yellow spot. In all programs, horticultural traits, adaptation, and resistance to tipburn are essential. New candidate sources of resistance were identified to race 2 isolates of Verticillium dahliae and pre-mature bolting. 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, powdery mildew, dieback, bacterial leaf spot, corky root, leafminer, tipburn, and pre-mature bolting. We initiated testing of all advanced breeding for shelf life after cutting harvested lettuce into salad. Testing was especially important for romaine-type cultivars with the TBSV resistance gene introgressed from the primitive lettuce accession PI 491224. Use of this accession in breeding programs frequently produces material with very short shelf life. Testing of lettuce for a rate of decay after processing allowed us to identify two advanced breeding lines with complete resistance to dieback and significantly improved shelf life. The two lines were released and seeds were provided to lettuce-breeding companies. We identified genes for resistance to TBSV and develop molecular markers for marker-assisted selection completed studies on the population developed from a cross between resistant cultivar Salinas and susceptible cultivar Valmaine. The results from linkage mapping were combined with the results from association mapping carried out on over 200 accessions from all horticultural types of lettuce. In collaboration with scientists from UC, Davis, we have mapped the position of the Tvr1 gene that confers resistance to dieback in lettuce and developed the PCR-based marker for marker-assisted selection. The marker was disseminated to the industry through a material-transfer agreement and feedback from the companies shows successful incorporation of this marker into all major breeding programs. We evaluated resistance in three lettuce populations to understand the molecular mechanism of field resistance to downy mildew, and collaboration with scientists from UC, Davis we developed the molecular linkage map based on SNP markers. The same mapping population is being analyzed with AFLP markers to saturate this map with molecular markers. Confirmed resistance in melon accessions TGR-1551 and PI 313970 to cucurbit yellow stunting disorder virus in Fall-planted melons.

4. Accomplishments
1. A single gene in La Brillante confers race 1 resistance to Verticillium wilt of lettuce. Verticillium wilt of lettuce, incited by the fungal pathogen Verticillium dahliae, is an emerging, devastating disease of lettuce production in central, coastal California that is discovered in more fields each year. This fungus can survive in the soil for more than 20 years in the absence of its host and soil fumigation provides only temporary relief from the disease. Host plant resistance is the most efficacious means for reducing losses to this soilborne pathogen. ARS researchers in the Crop Production and Protection Unit, Salinas, California, in collaboration with researchers at University of California, Davis and support from the California Leafy Greens Research Program, located a single gene on chromosome 9 of the lettuce genome that confers resistance to race 1 of the fungus causing verticillium wilt of lettuce. Other resistance (QTL) to Verticillium wilt was located on chromosomes 5 and 8. This discovery may lead to the development of molecular markers for this gene, which could accelerate the breeding of Verticillium wilt-resistant lettuce cultivars.

2. Release of a molecular marker selection of lettuce resistant to dieback disease. Dieback disease of lettuce is incited by the soilborne Tomato bushy stunt virus. The disease can be controlled by genetic resistance, but field-testing and breeding of lettuce for resistance to dieback disease is a slow, time-consuming process. ARS researchers in the Crop Production and Protection Unit, Salinas, California, California in collaboration with researchers at University of California, Davis and ARS researchers in Stoneville, Mississippi developed a PCR-based, molecular marker that is closely linked to the dieback resistance gene. Use of the marker in breeding dieback disease-resistant lettuce will substantially reduce the need for field-based testing; instead of several years of field-testing, a single DNA analysis permits virtually 100% detection of resistant lettuce plants. The marker was released to several lettuce breeding and biotechnology companies for selection of resistant material; the marker is used also in the ARS lettuce-breeding program in Salinas.

3. Identified a new threat to spinach production. Beet Necrotic Yellow Vein Virus (BNYVV) causes one of the most economically destructive diseases of sugarbeet known as rhizomania. Sugarbeet has not been widely grown in California for 10 years, but BNYVV and its vector can persist in soil for more than 20 years. Spinach had been known to be an experimental host of BNYVV in greenhouse tests, but ARS scientists at Salinas, California determined the virus can cause disease in experimental spinach fields and identified an aggressive strain of BNYVV infecting spinach in a commercial field in Ventura County, CA. Diseased spinach expressed symptoms of yellow-green or light-green vein clearing, mottling or yellow-green, chlorotic lesions on younger leaves as early as 28-days after planting, at the four to six leaf stage of growth; infected plants often became stunted, deformed, wilted, and died. There were significant differences in disease incidence and development among spinach cultivars and BNYVV was not transmitted through seeds. Research to develop a strategy to combat the disease is the next step.

Review Publications
Mou, B. 2009. Nutrient Content of Lettuce and its Improvement. Current Nutrition and Food Science 5:242-248.

Jenni, S., Hayes, R.J. 2010. Genetic variation, genotype x environment interaction, and selection for tipburn resistance in lettuce in multi-environments.. Euphytica, Feb 2010, Vol 171 (3), pages 427-439.

Mou, B., Ryder, E.J. 2010. MU06-857, a Green Leaf Lettuce Breeding Line with Resistance to Leafminer and Lettuce Mosaic Virus. HortScience. 45: 666-667.

Last Modified: 06/27/2017
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