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United States Department of Agriculture

Agricultural Research Service

Research Project: Genetic Enhancement of Lettuce, Spinach, Melon, and Related Species

Location: Crop Improvement and Protection Research

2013 Annual Report


1a.Objectives (from AD-416):
The focus of this research program is on quality traits, resistance 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. 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, spinach and melon.

Objective 1: Identify and select for novel sources of high-level host plant resistance in lettuce to priority diseases, insects, physiological defects and improved phytonutrient content. Improved lettuce germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.

Objective 2: Identify novel sources of host plant resistance in spinach to the new and emerging priority diseases, continue selection and improvement of host plant resistance to leaf miner, and elucidate genetic variation for low oxalic content to improve bioavailability of iron in fresh product. Improved spinach germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.

Objective 3: Identify novel sources of host plant resistance in melon to Cucurbit yellow stunting disorder virus and other priority diseases of melon, determine their genetic bases and evaluate potential for higher levels of resistance through pyramiding of non-allelic resistance genes. Improved cantaloupe and honeydew germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.


1b.Approach (from AD-416):
Objective 1: Collect, identify, characterize, and evaluate wild and unadapted germplasm of lettuce. Evaluate germplasm for new sources of resistance to viruses (lettuce dieback), fungi (downy mildew, lettuce drop, powdery mildew, Verticillium wilt), bacteria (bacterial leaf spot, corky root), and insects (lettuce aphid, leafminer). Evaluate lettuce for differential pre- and postharvest survival of E. coli O157 on lettuce germplasm. Improve phytonutrient content of lettuce. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.

Objective 2: Collect, identify, characterize, and evaluate unadapted germplasm of spinach. Evaluate germplasm for new sources of resistance to downy mildew and leafminer insect. Reduce oxalate content of spinach. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.

Objective 3: Collect, identify, characterize, and evaluate wild and unadapted germplasm of melon. Evaluate germplasm for new sources of resistance to Cucurbit yellow stunting disorder virus and powdery mildew. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.


3.Progress Report:
This is the first report for the project 5305-21220-005-00D initiated in April 2013, for additonial information see old project 5305-21220-004-00D. Specific research projects on lettuce, spinach and melon are summarized as follows: Survival of E. coli was tested in collaboration with a scientist in Albany, California, on healthy and downy mildew-infected leaves. Testing was performed in a laboratory on leaves with different degrees of downy mildew infection in high or low humidity environment. A field trial in Salinas was planted in Spring 2013. Three mapping populations were evaluated for their resistance to downy mildew. The populations originated from crosses of Grand Rapids x Iceberg, Salinas 88 x La Brillante, and Pavane x Parade, and will be used for mapping downy mildew resistance quantitative trait loci (QTL) in multiple environments. Ninety-six lettuce accessions were genotyped with high-resolution DNA melting approach to identify alleles associated with resistance to lettuce dieback. These accessions were previously phenotyped in field conditions. Bacterial leaf spot resistance in the lettuce cultivars La Brillante, Pavane, and Little Gem was determined to be conditioned by the same single dominant gene or closely linked genes. Leaf type F3 families were selected for improved lettuce drop resistance and horticultural characteristics. The lettuce drop resistance in a F7 romaine type breeding line was confirmed in a grower trial. Virescence in F2 PI 129535vir x Green Towers segregated as a single recessive gene. Plants that were slow bolting and virescent were selected for seed production and backcrossing to adapted romaine cultivars. Breeding lettuce for resistances to leafminers, corky root, yellow spot and herbicide, nutritional improvement, and horticultural traits was continued. Some advanced generation breeding lines are ready for release. Corky root and leafminer resistances were similar to or better than resistant controls, and their plant weight, core length, tipburn, and downy mildew resistance were comparable or better than control cultivars. Two field trials for drought tolerance in lettuce were conducted. Some varieties had no significant yield reduction even with a 50% reduction of irrigation water. Breeding spinach for resistances to downy mildew, leafminers and herbicide, as well as horticultural traits by using a recurrent selection method was continued. Spinach downy mildew isolates for germplasm screening and breeding were collected, preserved, and increased. The screening of the USDA spinach germplasm collection was completed and accessions with resistance to Race 1 and Race 2 of Verticillium wilt pathogen were found.


Last Modified: 10/22/2014
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