Location: Crop Improvement and Protection Research
2015 Annual Report
Objectives
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.
Approach
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.
Progress Report
Lettuce dieback disease (caused by two closely related Tombusviridae) is widespread in all types of lettuce with the exception of modern iceberg-type cultivars. We tested 192 lettuce accessions for resistance to dieback and analyzed them for molecular markers to identify marker-trait associations. Five of the highly resistant accessions identified were selected for genetic studies.
Many lettuce cultivars are susceptible to downy mildew caused by the oomycete, Bremia lactucae. We identified at least five loci that confer resistance to downy mildew in the lettuce cultivar La Brillante. A new, dominant gene (Dm50) that confers complete resistance to specific isolates of downy mildew was detected, but the gene is ineffective against downy mildew isolates prevalent in California and The Netherlands. A quantitative trait locus (QTL) located at the Dm50 chromosomal region was detected, and four additional QTL for resistance to B. lactucae were identified. The QTL will facilitate breeding of downy mildew resistant lettuce.
Lettuce drop disease resistance breeding: We crossed drop-resistant lettuce breeding lines selected from crosses of two different populations developed by crossing unique sources of resistance to lettuce drop. All of the selected lines used in the crosses had improved lettuce drop resistance, but none had all the desired horticultural traits; four populations were selected for further testing. In addition, we evaluated 48 advanced romaine type breeding lines from a third source of drop resistance which were evaluated for drop resistance, yield, and quality. Eight lines were selected from this population for additional testing. We will likely release some or all of these lines in 2016.
Lettuce drop genetics: We are developing an advanced genetic population of lettuce from the cross Batavia Reine de Glaces x Eruption to study the genetics of resistance in ‘Eruption’. Resistance will be evaluated in field experiments in 2016. Plant material will be given to collaborators by the end of 2015 for generation of molecular marker data and construction of a genetic linkage map of the cross.
Lettuce Fusarium wilt: This disease has become increasingly important in the Yuma, Arizona area. The pathogen is present in Salinas Valley, California, where it has not been a problem due, most likely to cooler temperatures near the coast, but has been suspected/implicated in crop loss in the south end of Salinas Valley, where Verticillium wilt race 1 may be present in some fields. We initiated work this year to combine Fusarium wilt and Verticillium wilt race 1 resistances for south Salinas Valley.
Lettuce powdery mildew: Recent emphasis has been on establishing and maintaining isolates in axenic culture for controlled inoculation tests. Isolates were obtained from three different areas in the Yuma production area and Salinas. The isolates were partially characterized on the set of lettuce powdery mildew race differentials established by Czech researchers.
Lettuce tipburn romaine breeding: We field tested 11 romaine breeding lines from the cross Green Towers × Salinas for tipburn resistance in Yuma; all had less tipburn than the romaine cultivar Green Towers. Core (stem) heights and head weights of the lines were similar to Green Towers, though they were horticulturally less desirable.
Virescent romaine lettuce: The virescent trait is useful for development of “open” romaine lettuce heads that will be less susceptible to tipburn disorder and yet remain light yellow-green, which is desirable for lettuce hearts. We are producing from 200+ virescent selections for use in 2016 field experiments.
Verticillium wilt race 2 poses a serious threat to lettuce production in Salinas Valley and other California coastal lettuce production areas. We have been working with several sources of partial resistance, and are combining them through intercrossing in an effort to pyramid the many unique genes to achieve a higher level of resistance than any one source can provide alone.
Resistance of lettuce to leafminers, corky root, yellow spot, and herbicides, and nutritional improvement: We continued crossing, selection, and seed increase for resistances to leafminers, corky root, yellow spot, and herbicide, nutritional improvement, appearance, and horticultural traits. We also continued our new project to breed leafminer resistance for baby lettuce used in spring mix. Breeding lines in advanced generations were tested in field trials, and some lines are ready to be released. The corky root and leafminer resistance of the breeding lines were similar to or better than resistant controls, while their plant weight, core length, tipburn, and downy mildew resistance were comparable or better than control cultivars.
Resistance of spinach to downy mildew, leafminers, and herbicide: We made crosses and selections to breed for resistances to downy mildew, leafminers, and herbicide, as well as horticultural traits by using a recurrent selection method. We continued to test spinach varieties for cadmium content. We also continued spinach genomic studies with collaborators at three universities.
Melon: Resistance to Cucurbit yellow stunting disorder virus (CYSDV): We continued field evaluation for resistance to CYSDV in populations segregating from crosses with three sources of resistance. We determined that resistance in TGR 1551 is recessive, not dominant as was originally reported by Spanish investigators. Resistance was also determined to be recessive in several other melon accessions from India.
Resistance to sweet potato whitefly (SPWF): Melon production in the Sonoran desert of California, Arizona and western Mexico are devastated by adverse effects of SPWF feeding. Eight potential sources of low-level resistance to SPWF have been reported, but in several naturally infested field tests they are not consistently different from commercial cultivars for numbers of adults, eggs or immature life stages. A unique and potential new source of resistance to SPWF was observed in fall 2014 and is undergoing further evaluation and comparison with the previously reported sources of resistance in a replicated field test and controlled infestation greenhouse tests that are underway.
Accomplishments
1. New source of resistance to bacterial leaf spot of lettuce. Bacterial leaf spot reduces yield and shortens the shelf life of all types of lettuce. The disease occurs throughout the U.S., including the Salinas Valley in California where lettuce production is worth more than $1 billion. ARS researchers in Salinas, California, identified new sources of resistance to a strain of the disease that defeats the only reported resistance gene, Xar1. This is an important discovery, since novel plant genetic variation is the basis of plant breeding and genetics research. The newly found resistance prevents disease by this resistance-breaking strain of bacterial leaf spot, and, thereby, increases the sustainability of lettuce production.
2. Identification of Verticillium wilt-resistant spinach varieties. Wilt disease caused by Verticillium dahliae has not presented a problem in California spinach production because the crop is harvested well before the symptoms develop during the post-stem elongation (bolting) stage. However, infested spinach seeds introduce or increase inoculum in the soil for rotational crops such as lettuce and strawberry. ARS researchers at Salinas, California, in collaboration with researchers at University of California, screened the U.S. spinach germplasm collection and found resistant varieties against two races of the pathogen. The sources of resistance identified in this study are useful for spinach cultivar improvement.
3. Mapped novel resistance genes to downy mildew in lettuce cultivar La Brillante. Many cultivars of lettuce (Lactuca sativa L.), the most popular leafy vegetable, are susceptible to downy mildew disease caused by Bremia lactucae. ARS researchers at Salinas, California, in collaboration with scientists from the University of California in Davis, California and KeyGene in The Netherlands identified genes contributing to resistance against downy mildew in cv. La Brillante. The presence of a new dominant resistance gene (designated Dm50) that confers complete resistance to specific isolates was detected in laboratory tests of seedlings inoculated with multiple diverse isolates. Additional quantitative trait loci for resistance were detected in three chromosomes.
4. Downy mildew disease promotes the colonization of lettuce with human enteric pathogens. ARS researchers in Salinas and Albany, California, demonstrated a positive interaction between a plant pathogen and enteric pathogens on lettuce. They demonstrated that Bremia lactucae, the oomycete that causes downy mildew disease of lettuce, significantly alters the behavior of enteric pathogens on lettuce leaf surfaces, thereby leading to a higher survival and multiplication rate of Escherichia coli (E. Coli) and Salmonella enterica (S. enterica) on lettuce leaves. This finding shows for the first time a significant interaction between downy mildew and survival of human enteric pathogens on lettuce. Therefore development of lettuce breeding lines with high resistance to downy mildew is highly desirable for both the quality of the final product (less blemishes) and the product safety (less likely survival of E. coli and S. enterica). News outlets on at least three continents reported results of this work.
5. Detection of decay in fresh-cut lettuce using next-generation phenotyping. ARS researchers from Salinas, California collaborated with scientists at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Canberra, Australia, to develop the first method for automated detection of decay in salad-cut lettuce held in modified atmosphere plastic bags through the use of hyperspectral analysis and chlorophyll fluorescence imaging. The method was 97% accurate when tested on lettuce leaves that ranged in color from light yellow, to green, dark green, to dark red. These are the first reported methods that use hyperspectral analysis and chlorophyll fluorescence for post-harvest evaluation of fresh-cut lettuce. The non-destructive nature of the methods allows rapid and repeated evaluation of samples over time and presents the opportunity for development of a commercial, high throughput scanner for evaluation of bagged, salad-cut lettuce quality. This technique will facilitate breeding lettuce for longer shelf life.
Review Publications
Knepper, C.P., Mou, B. 2015. Semi-high throughput screening for potential drought-tolerance in lettuce (Lactuca sativa) germplasm collections. Journal of Visualized Experiments. (98):e52492. doi: 10.3791/52492.
Hayes, R.J., Trent, M., Truco, M., Antonise, R., Michelmore, R.W., Bull, C.T. 2014. The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars. Horticulture Research. 1:14066.
Mou, B. 2015. Red leaf lettuce breeding line with resistance to corky root, 06-810. HortScience. 50:143-144.
Samtani, J.B., Rachuy, J.S., Mou, B., Fennimore, S.A. 2014. Evaluation of tribenuron-methyl on sulfonylurea herbicide tolerant lettuce germplasm. Weed Technology. 28(3):510-517.
Simko, I., Zhou, Y., Brandl, M. 2015. Downy mildew disease promotes the colonization of romaine lettuce by Escherichia coli O157:H7 and Salmonella enterica. BMC Microbiology. 15:19. doi: 10.1186/s12866-015-0360-5.
Gurung, S., Short, D.P., Hu, X., Sandoya, G., Hayes, R.J., Koike, S., Subbarao, K.V. 2015. Host range of Verticillium isaacii and Verticillium klebahnii from artichoke, spinach, and lettuce. Plant Disease. doi: 10.1094/PDIS-12-14-1307-RE.
Gurung, S., Short, D.P., Hu, X., Sandoya, G., Hayes, R.J., Subbarao, K.V. 2015. Screening of wild and cultivated Capsicum germplasm reveals new sources of Verticillium wilt resistance. Plant Disease. doi: 10.1094/PDIS-01-15-0113-RE.
Hu, X., Gurung, S., Short, D.P., Sandoya, G., Shang, W., Hayes, R.J., Subbarao, K.V. 2015. Non-defoliating and defoliating strains from cotton correlate with races 1 and 2 of Verticillium dahliae. Plant Disease. doi: 10.1094/PDIS-03-15-0261-RE.
Short, D., Sandoya, G., Vallad, G., Koike, S., Xiao, C., Wu, B., Gurung, S., Hayes, R.J., Subbarao, K. 2015. Dynamics of Verticillium species microsclerotia in field soils in response to fumigation, cropping patterns, and flooding. Phytopathology. 105:638-645.
Batuman, O., Natwick, E.T., Wintermantel, W.M., Tian, T., Mccreight, J.D., Hladky, L.L., Gilbertson, R.L. 2015. First report of an ipomovirus infecting cucurbits in the Imperial Valley of California. Plant Disease. doi: 10.1094/PDIS-12-14-1248-PDN.
Simko, I., Jimenez-Berni, J.A., Furbank, R.T. 2015. Detection of decay in fresh-cut lettuce using hyperspectral imaging and chlorophyll fluorescence imaging. Postharvest Biology and Technology. 106:44–52.
Simko, I. 2015. Analysis of bibliometric indicators to determine citation bias. Palgrave Communications. doi: 10.1057/palcomms.2015.11.
Simko, I., Ochoa, O.E., Pel, M.A., Tsuchida, C., Forcada, C.F., Hayes, R.J., Truco, M.J., Antonise, R., Galeano, C.H., Michelmore, R.W. 2015. Resistance to downy mildew in lettuce ‘La Brillante’ is conferred by dm50 gene and multiple QTL. Phytopathology. 105:1220-1228.
