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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

2012 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 (Bacterial Leafspot, corky root) 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.

3. Progress Report:
Lettuce. Obtained 58 CGN accessions reported to have resistance to European strains of lettuce aphid biotype(s) 0 and/or 1 for testing against California strains of biotype 0. Research was conducted on breeding and genetics of V. dahliae, lettuce drop, bacterial leaf spot, and shelf-life in 17 greenhouse, laboratory, or field experiments. Iceberg lettuce breeding lines possessing the Verticillium resistance 1 gene were selected with improved horticultural characteristics. Romaine lettuce breeding lines that incorporate lettuce drop resistance from ‘Eruption’ were developed with improved resistance and quality. Seventeen advanced iceberg, romaine, and leaf breeding lines that possess resistance to four lettuce diseases were evaluated in three replicated experiments for 14 horticultural, pest resistance, and post-harvest characters. Rapid decay of packaged salad-cut lettuce in two diverse lettuce cultivars was determined to be conditioned by one major QTL and two small effect QTL. Bacterial leaf spot resistance of ‘La Brillante’ and ‘Little Gem’ was determined to be conditioned by a single gene. Developed two romaine breeding lines with resistance to dieback and long shelf life after chopping for salad. Developed and evaluated five mapping populations for their reactions to downy mildew in order to locate resistance genes on the molecular linkage map. Tested 1,500+ accessions for resistance to downy mildew to elucidate molecular mechanisms of quantitative disease resistance, and crossed lines with high levels of downy mildew resistance to initiate development of breeding lines with high levels of quantitative resistance. Developed a new formula termed the area under the disease progress steps to combine mildew disease data across multiple trials and new statistical approaches to integrate plant data from multiple trials. Continued to breed for combined resistances to leafminer, corky root, yellow spot and herbicide, nutritional improvement, and horticultural traits, and resistance to leafminer in baby lettuce. Levels of corky root and leafminer resistances exhibited by advanced-generation breeding lines were similar to the resistant sources, while their plant weight, core length, and tipburn and downy mildew resistances were comparable or better than the control cultivars. Spinach. Continued to breed for resistances to downy mildew, leafminers, and herbicide, as well as horticultural traits. Selected plants were transplanted into isolators to produce seeds for further selection. We continued to collect, preserve, and increase downy mildew isolates for screening germplasm for resistance and breeding work. Melon. Selected for uniform resistance to Cucurbit yellow stunting disorder virus in lines derived from two accessions and completed an allelism test of their resistances in Spain. Confirmed resistance in a third accession and identified putative resistance in nine accessions to the virus. Completed evaluation of 430 accessions to seven races of powdery mildew. Evaluated Turkmen accessions for adaptation & fruit quality in Imperial Valley, Califorina.

4. Accomplishments
1. Genetics of salad-cut lettuce shelf-life. Packaged salads are an important market for lettuce producers and breeding lettuce with commercially acceptable or extended shelf-life is needed to sustain this industry. ARS researchers in Salinas, CA, determined the genetics of decay using populations derived from crosses of ‘Salinas 88’ with ‘La Brillante’ and ‘Pavane’ with ‘Parade’. Rapid decay was inherited from ‘La Brillante’ and ‘Pavane’, and a major quantitative trait locus (QTL) describing 30 to 80% of the differences (depending on the experiment) in decay was located on chromosome 4 in both crosses. The QTL was detected in lettuce grown in different years, locations, and packaging atmospheres. Smaller QTL were detected on chromosomes 1 and 9. Breeding for reduced decay is feasible since the trait is conditioned by few QTL with minimal genotype x environment interaction.

2. Use of lettuce sensitivity to triforine as a selectable marker for detecting hybrids. The current and most frequent method of cross-pollination in lettuce breeding programs involves washing the recipient flowers to remove the pollen followed by transfer of the pollen from donor flowers, but results of emasculation can be quite variable and self-fertilization may occur. Accurate testing of F1 hybrids is, therefore, necessary to identify and eliminate offspring resulting from self-pollination. We have developed an assay that uses plant reaction to triforine (a compound found in a variety of commercial systemic fungicides) as a marker for detecting hybrids. These hybrids have to originate from a cross between parents with different responses to triforine treatment. The triforine-based assay allows inexpensive, rapid, and accurate detection of hybrid material originating from a cross between phenotypically similar parents. Description of the assay and results were published in a peer-reviewed journal and the assay is currently used in our breeding program.

Review Publications
Simko, I., Hayes, R.J., Truco, M.J., Michelmore, R.W. 2011. Mapping dominant negative mutation for triforine sensitivity in lettuce and its use as a selectable marker for detecting hybrids. Euphytica. 182(2):157-166.

Mou, B. 2012. Mutations in Lettuce Improvement. International Journal of Plant Genomics. doi:10.1155/2011/723518.

Mou, B., Richardson, K.L., Benzen, S.D., Liu, H. 2012. Effects of beet necrotic yellow vein virus in spinach cultivars. Plant Disease. 96:618-622.

Mou, B. 2011. Green leaf lettuce breeding lines with resistance to corky root, 06-831 and 06-833. HortScience. 46:1324-1325.

Mou, B. 2011. Improvement of horticultural crops for abiotic stress tolerance. An Introduction. HortScience. 46(8):1068-1069.

Mou, B., Wang, G. 2012. Asia’s indigenous horticultural crops: an introduction. HortScience. 47:819-820.

Hayes, R.J., Maruthachalam, K., Vallad, G.E., Klosterman, S.J., Subbarao, K.V. 2011. Selection for resistance to Verticillium wilt caused by race 2 isolates of Verticillium dahliae in accessions of lettuce (Lactuca sativa L.).HortScience. 46(2):201-206.

Mayton, H., Rauscher, G., Simko, I., Fry, W. 2011. Evaluation of the RPi-ber late blight resistance gene for tuber resistance in the field and laboratory. Plant Breeding. 130:464-468.

Simko, I., Piepho, H. 2012. The area under the disease progress stairs: calculation, advantage, and application. Phytopathology. 102:381-389.

Simko, I., Eujayl, I.A., Van Hintum, Jl, T. 2012. Empirical evaluation of DArT, SNP, and SSR marker-systems for genotyping, clustering, and assigning sugar beet hybrid varieties into populations. Plant Science. 184:54-62.

Last Modified: 06/26/2017
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