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

Location: Crop Improvement and Protection Research

2009 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
Lettuce objectives are primarily to incorporate resistances to several diseases, insects, and physiological defects into iceberg, romaine, and mixed lettuces for coastal and desert production and also to increase nutritional content of lettuce. Laboratory, greenhouse and field experiments were conducted in the coastal and desert valleys of California, and Yuma, Arizona to develop lettuce resistant to downy mildew, lettuce big vein disease, lettuce drop/Sclerotinia, Verticillium wilt, Fusarium wilt, lettuce dieback/tombusviruses, corky root, leafminer, lettuce aphid, and tipburn. We confirmed resistance in germplasm to Verticillium wilt, Fusarium wilt and lettuce aphid. We identified new sources of resistance to Verticillium dahliae race 2, and extended shelf life in modified atmosphere packaging. Selections were taken from breeding populations, advanced breeding lines were evaluated, and genetic studies were conducted, as part of breeding for resistance to the various problems identified above. We are developing molecular markers linked to genes conferring resistance to Verticillium wilt, leafminer, and dieback, tipburn. We developed 61 microsatellite markers based on expressed sequence tags (EST); the first publicly available microsatellite markers for use in lettuce population studies. We developed and mapped more than 150 anonymous microsatellite markers from genomic DNA. We mapped the position of the Tvr1 gene that confers resistance to dieback in lettuce. Two advanced breeding lines with complete resistance to dieback and improved shelf-life were identified for release. For melon, we continue to pursue resistance to Cucurbit yellow stunting disorder virus (CYSDV) and cucurbit powdery mildew caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea), and we are in the final phase of development of western U.S. shipping type melon germplasm with fractal (multiple lateral) plant architecture and monoecious sex expression. CYSDV first occurred in Arizona, California and western Mexico in Fall 2006, but has since become widely prevalent in Spring plantings and poses the most acute threat to melon production in these areas. We confirmed resistance to CYSDV in PI 313970 and have determined its inheritance to be recessive in naturally-infected field tests. We also confirmed resistance to CYSDV in TGR-1551 (a.k.a. PI 482420) reported to have a single dominant gene for resistance to CYSDV though there is contradictory evidence. We are prepared to determine the allelism of resistances in these two sources (PI 313970 and TGR-1551) of resistance to CYSDV in melon. We continued to evaluate Indian melon germplasm for new sources of resistance to CYSDV and other germplasm for testing in Fall 2009 and Spring 2010. Cucurbit powdery mildew is considered by many commercial seedsmen to be the most serious disease of melons worldwide as it is always present and the number of physiological races is apparently limitless. We confirmed resistance to P. xanthii race S to be conditioned by a recessive gene that is epistatic to two recessive genes for resistant blister (non-race specific resistance) reaction in PI 313970.

4. Accomplishments
1. Identified a gene for Verticillium wilt resistance in lettuce. Verticillium wilt is a relatively new and potentially devastating disease of lettuce in central, coastal California. ARS researchers at Salinas, California with university collaborators and industry funding identified a single gene in the lettuce variety La Brillante that confers resistance to Verticillium wilt race 1 and located it on chromosome 9 of the lettuce genome. Such germplasm is also useful for determining which race is present in any given field. This discovery enables development of molecular markers, which could accelerate the breeding of Verticillium wilt race 1-resistant lettuce cultivars.

2. New source of genes for downy mildew and leafminer resistance in lettuce. Downy mildew disease and leafminer insects are major problems on lettuce worldwide. A researcher at Salinas, California with industry funding determined that PI 509525, a weedy relative of lettuce, is resistant to leafminers and is a non-host to downy mildew. Experiments conducted over four years showed each resistance controlled by an independent, single gene. Identification of the resistance genes permits transfer of resistances to leafminers and downy mildew from the wild species into cultivated lettuce. Reduced damage by leafminers and complete resistance to downy mildew will help growers increase yields and quality of lettuce.

3. Molecular markers linked to dieback resistance in lettuce. Testing lettuce for resistance to dieback disease, caused by a soil borne virus, can take several years. An ARS researcher at Salinas, California with industry funding developed markers closely linked to the dieback resistance gene. This development reduces the time to identify resistant individuals from several years to a single, lab-based DNA analysis, accurately identifies resistant plants, and distinguishes heterozygous individuals from homozygous individuals whose offspring will breed true for resistance in the following generation. The marker is being used to select superior dieback resistant materials in the Salinas breeding program. Lettuce breeding companies are incorporating these markers into their breeding program, as well.

4. Disease resistant lettuce germplasm released. Bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians is an important but sporadic disease of lettuce in California. ARS researchers at Salinas, California with industry funding released six breeding lines with good head quality and resistance to Bacterial leaf spot. This germplasm will reduce yield losses to this diseaes at harvest and postharvest losses of whole head and chopped lettuce products. The material is currently being used by private and public breeders for development of proprietary varieties that are resistant to this disease.

5. New source of resistance in melon to Cucurbit yellow stunting disorder virus. Cucurbit yellow stunting disorder virus, a whitefly-transmitted virus is the most acute threat to melon production in southern parts of the U.S. from California to Florida where whiteflies are prevalent. ARS researchers at Salinas, California with industry funding determined that resistance to Cucurbit yellow stunting disorder virus is controlled by one or more recessive genes in PI 313970, a non-dessert type melon from India. Resistance in PI 313970 is not immunity (freedom from infection) but symptoms are mild when compared with susceptible varieties, and may be used to strengthen resistance from TGR-1551, a melon from Zimbabwe. This information is being used by seedsmen to develop CYSDV-resistant melons for the desert southwest U.S. and Mexico.

Review Publications
Mayton, H., Griffiths, H., Simko, I., Cheng, S., Lorenzen, J., De Jong, W., Fry, W.E. 2009. FOLIAR AND TUBER BLIGHT RESISTANCE IN A SOLANUM TUBEROSUM BREEDING LINES.. Plant Breeding.DOI:10.1111/J.1439-0523.01671.x

Mou, B. Evaluation of Oxalate Concentration in the U.S. Spinach Germplasm Collection. HortScience 43(6):1690-1693.2008.

Mou, B. Screening for resistance to leafminer in spinach. HortScience Vol. 43(6) October 2008.

Mou, B., Koike, S.T., Du Toit, L.J. Screening for Resistance to Leaf Spot Diseases of Spinach.. HortScience 43(6):1706-1710.2008.

Cuevas, H.E., Staub, J.E., Simon, P.W., McCreight, J.D., Zalapa, J.E. 2008. Mapping of Genetic Loci that Regulate Accumulation of Beta-Carotene in Fruit of U.S. Western Shipping Melon (Cucumis Melo L.) and Their Association With Putative Carotenoid Biosynthesis Genes. Theoretical and Applied Genetics 117:1345-1359.

Sedlarova, M., Lebeda, A., Miksikova, P., Duchoslav, M., Sedlakova, B., Mccreight, J.D. 2009. Histological Aspects of Cucumis melo PI 313970 Resistance to Podosphaera xanthii and Golovinomyces cichoracearum. Journal of Plant Diseases and Protection 116: 169-176.

Paris, M.K., Zalapa, J.E., Mccreight, J.D., Staub, J.E. 2008. Genetic Dissection of Fruit Quality Components in Melon (Cucumis melo L.) using a RIL Population Derived from Exotic x Elite US Western Shipping Germplasm. Molecular Breeding 22:405-419.

Simko, I. 2009. Development of EST-SSR Markers for the Study of Population Structure in Lettuce (Lacutca sativa L.). Journal of Heredity 180: 256-262.