Location: Crop Improvement and Protection Research2011 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
Partial resistance to lettuce aphid resistance in PI 491093 was again expressed in field studies. Evaluated reactions of four lettuce populations to Bremia lactucae to understand molecular mechanism of field resistance to downy mildew. We developed, in collaboration with scientists from University of California, Davis and Keygene (The Netherlands), molecular linkage maps based on combination of single nucleotide polymorphism (SNP) and AFLP (amplified fragment length polymorphism) markers. Seeds from the mapping populations were distributed to collaborators and will be analyzed for resistance to downy mildew in multiple environments. Crossing, selection, and seed increase to breed for resistances to leafminers, corky root, yellow spot, and herbicide, nutritional improvement, and horticultural traits. New project to breed leafminer resistance for baby lettuce used in spring mix. Breeding lines in advanced generations were tested for possible release. Cucurbit powdery mildew (CPM) races on melon were monitored in Spring 2011 in a naturally infected field test of 13 commonly used melon race differentials and 16 other known melon sources of CPM resistance. CPM race S was widespread in commercial melon fields in the desert southwest U.S. in 2003 and was shown subsequently to infect all previously reported sources of resistance. Race 1 was found in Spring 2011 despite the widespread deployment of genes for resistance to it in the current melon hybrid cultivars. Abundant chasmothecia of cucurbit powdery mildew were observed for the first time on squash (Cucurbita pepo) plants in a greenhouse at Salinas, CA in winter 2011. Morphological and molecular data confirmed the pathogen on melon, cucumber and squash as Podosphaera fuliginea. Two isolates of Golovinomyces cichoracearum (Gc) found infecting lettuce in a greenhouse failed to infect cucumber or melon, and may therefore may be regarded as representatives of G. cichoracearum sensu stricto, which is restricted to members of the Asteraceae. Host-parasite interactions observations from microscopic observations clearly indicated cucumber to be a non-host. These observations confirmed earlier study of field-collected isolate of G. cichoracearum from lettuce, and general lack of infection by G. cichoracearum in any part of the U.S. Continued to pursue the characterization of host plant resistance to CYSDV in PI 313970 as well as in TGR-1551, a source reported resistant in Spain. TGR-1937, reported to be moderate source of resistance, is being investigated as a potential third unique source of resistance to CYSDV in melon. Screened ca. 100 melon accessions from India in naturally infected field tests for new sources of CYSDV resistance. Spinach crosses and selections for resistances to downy mildew, leafminers, and herbicide, as well as horticultural traits. Spinach downy mildew isolates collected and preserved for screening germplasm for resistance and breeding.
1. Sclerotinia resistance in lettuce. Lettuce production in California and Arizona is significantly impacted by the disease lettuce drop, caused by the soil borne fungi Sclerotinia sclerotiorum and Sclerotinia minor. Resistant cultivars could reduce losses, but physiological resistance that is independent of plant architecture must be identified in order to breed these cultivars. ARS researchers in Salinas, CA identified lettuce germplasm that possess partial resistance to lettuce drop. Importantly, the resistance appears to be physiologically based. Development of this resistance is important for lettuce production in California and Arizona.
2. New approaches for combing and analyzing data from non-uniform tests. Plant breeders evaluate large arrays of diverse germplasm (e.g., varieties, breeding lines, and exotic accessions) in numerous tests over many years and locations. Ordinal rating scales (e.g., 1 to 9) are often used to evaluate specific traits that may differ over time and location. Combining data from two or more tests is complicated because the rating scales may not be identical (1 to 5 versus 1 to 3). Moreover, the numbers of accessions in each test often differ, and usually only a few of them are represented in many of the tests. An ARS researcher in Salinas, CA identified statistical approaches that allow the combining and analyses of data from ordinal rating scales in multiple tests. The ability to combine data from different tests has important consequences for plant breeders because it allows more accurate estimates of plant performance and selection of germplasm with the most desirable traits.
3. Mechanisms of resistance to leafminers and aphids in lettuce identified. Leafminer (Liriomyza langei) and aphid (Myzus persicae, Macrosiphum euphorbiae and Aulacorthum solani) are important insect pests of lettuce. ARS researchers at Salinas, CA investigated biochemical aspects of lettuce host plant resistance to these pests in growth chambers and fields. Resistances to leafminers and aphids were significantly associated with contents of sucrose, glucose, flavonoids, chicoric acid, chlorogenic acid, rutin, protein, carotene, lutein, and chlorophyll in lettuce. This study provides insights into mechanisms of resistance to leafminers and aphids as well as screening and selection tools for breeding insect-resistant lettuce.
Hayes, R.J., Wu, B., Subbarao, K.V. 2011. A single recessive gene conferring short leaves in romaine x Latin type lettuce (Lactuca sativa L.) crosses, and its effect on plant morphology and resistance to lettuce drop caused by Sclerotinia minor Jagger. Plant Breeding. 130:388-393.