Location: Vegetable Research2018 Annual Report
1. Identify and characterize host plant resistance genes and develop germplasm lines of sweetpotato and watermelon that are resistant or tolerant to economically important insect pests of important vegetable crops, and develop germplasm lines adapted to low input, sustainable production systems [NP304, Component 3, Problem Statement 3A2]. 1.A. Characterize watermelon germplasm lines with resistance to the sweetpotato whitefly and incorporate resistance factors into advanced watermelon breeding lines. 1.B. Identify and characterize resistance genes and genotypes of sweetpotato with resistance to soil insect pests, elucidate mechanisms of pest resistance, and develop germplasm clones that are resistant to soil insect pests and have good horticultural characteristics. 1.C. Identify sweetpotato clones tolerant of weed interference and/or whitefly-transmitted viruses that are superior to conventional cultivars for organic and sustainable production. 2. Develop methods to improve control of insect pests, especially whiteflies, in vegetable production systems, and identify the effects of biotic and abiotic factors on populations of pests and their biological control agents, and on whitefly:host plant:virus interactions. This objective will be enhanced by developing and applying novel genomics-based technologies to manage whiteflies and whitefly-transmitted viruses in vegetable crops. 2.A. Determine the effect of biotic and abiotic factors on populations of biological control agents of whiteflies in vegetable production systems. 2.B. Determine the impact of factors associated with climate change on whitefly:host plant:virus interactions and whitefly endosymbionts. 2.C. Investigate sustainable management approaches for pests in vegetable crops, including detection of pest populations such as pickleworms.
Conduct laboratory, greenhouse and field experiments to identify sources of resistance and evaluate genetic populations to determine resistance against the sweetpotato whitefly in watermelon and against soil insect pests, weeds and whitefly-transmitted virus in sweetpotato. Assay chemical and physical mechanisms of resistance to pests using gas chromatography-mass spectrometry (GC-MS), portable “electronic nose,” Y-tube olfactometers, and other assays. Use PCR-markers and other genomic technologies, such as genotype by sequencing, to identify sequences linked to the studied characters and to locate controlling genes on linkage maps. Cross appropriate germplasm to facilitate the incorporation of resistance into advanced breeding lines or new cultivars. Assess the competitive advantage against weeds of sweetpotato genotypes with more vigorous growth habits in comparison to less competitive conventional cultivars, identify competitive genotypes with good horticultural quality, and evaluate them as a component in integrated management systems for conventional and organic growers. Use a recurrent mass selection breeding approach to generate sweetpotato clones with high levels of resistance and good horticultural characteristics. Continue ongoing searches for new resistances or tolerances among watermelon and sweetpotato accessions from the U.S. Plant Introduction System and other collections. Make improved germplasm available for use by the vegetable industry. Investigate the influence of climate and biotic factors on insect populations by using environmental chambers and field cages. Assess the behavior and ecology of pickleworms and other pests for their control by the development of new formulations and ratios of the pheromone components and testing them in flight tunnel and field environments. Study the epidemiology of whitefly-transmitted Sweet potato leaf curl virus in sweetpotato using biological assays and molecular detection techniques, including real-time (RT)-PCR and quantitative (q)PCR.
Development of new sweetpotato germplasm was continued with over 45,000 seeds harvested from two open-pollinated breeding field plots. Selection for improved sweetpotato germplasm was continued; over 11,000 seedlings were evaluated in the field. Over 5,900 of these seedlings were evaluated and 112 selections were made for sweetpotato clones that exhibited compact growth and reduced light penetration through the canopy; these will be evaluated further in weed competition studies. Selections (14) made in 2016 and 2017 for compact plant growth habit and insect resistance were established into field trials for selection for the most competitive sweetpotato clones against weeds. Furthermore, over 150 2nd year seedlings, intermediate, advanced, and regional sweetpotato clones were evaluated in field plots for insect resistance and for other important horticultural traits. Three breeding fields were established for continued development of new sweetpotato germplasm. Genetic analysis of the USDA, ARS Sweetpotato Germplasm Collection continued and half of the collection was sent to university collaborators for genotyping by sequencing. Research was initiated on screening 50 sweetpotato plant introductions from the USDA, ARS Sweetpotato Germplasm Collection to identify sources of resistance to a rising problem by a root-knot nematode species (Meloidogyne enterolobii). Research was continued on species-level identification of the click beetle complex that attacks sweetpotato. Biotic and abiotic factors affecting abundance, spatial distribution and damage to sweetpotato roots caused by this beetle were further evaluated. Research was continued on the effects of sweetpotato cultural practices (including plasticulture mulch and irrigation practices) on soil arthropod abundance and pest damage to storage roots. Research was also initiated to evaluate the effectiveness of different baits in soil arthropod traps. Over 40 sweetpotato plant introductions were selected from the USDA-ARS Sweetpotato Germplasm Collection and are being evaluated for tolerance to the whitefly-transmitted Sweet Potato Leaf Curl Virus (SPLCV). Leaf tissue from over 5,800 sweetpotato seedlings were tested for seed transmission of SPLCV and this research supports that there is a lack of seed transmission of SPLCV. Research was continued on plant resistance mechanisms and the development of plant resistance in watermelon against whiteflies. Research was continued on the assessment of climate and other factors affecting whiteflies and a whitefly predator (Delphastus catalinae). A genetic (transcriptome) analysis was conducted on the whitefly Bemisia tabaci on tomato infected with the Tomato yellow leaf curl virus, and whitefly genes were identified that may be responsible in regulating the ability of the whitefly to acquire and transmit this virus. Through financial support from the pickle industry, screening of cucumber for resistance to pickleworm continued, with over 500 plant introductions planted into field plots to identify sources of resistance. Field and laboratory studies were continued to isolate, identify, and characterize insect-produced chemicals that affect the reproductive behavior of insect pests of sweetpotato (i.e., click beetles). Field trials with click beetle sex pheromone were begun to evaluate dose-response rates and release techniques. Studies aimed at the development of improved detection and monitoring methodologies of vegetable pests were initiated by incorporation of light cues with sex pheromones. Colonies of several species were established from field-collected insects; these insects were used in studies to evaluate host finding and reproductive behavior in response to sources of resistance. Studies continued to evaluate the effect of cover crops on soil arthropod species composition and abundance during transition from conventional to organic production practices.
1. Improvement of sweetpotato. Improvement of sweetpotato through plant breeding is difficult because of the complex genetics and high variability within the crop. Genomic data can be used to determine genetic relationships among parental lines and to identify sources of genetic variation associated with resistance and tolerance to pests, diseases, environmental stress, and other high value traits of the plants. There is scarce knowledge about the level of genetic diversity within the USDA, ARS sweetpotato germplasm collections. ARS researchers at Charleston, South Carolina, in collaboration with university researchers in North Carolina and Tennessee assessed population structure and genetic diversity of over 400 sweetpotato types from the USDA, ARS germplasm collections. Sweetpotato clones originating from 8 broad geographical regions (Africa, Australia, Caribbean, Central America, Far East, North America, Pacific Islands, and South America) were evaluated and characterized with DNA markers using a genome sequencing protocol that was optimized for complex genetic species like sweetpotato. The results indicate that there is high genetic diversity within the U.S. sweetpotato collections, and this is the first large scale analysis of genetic diversity of this germplasm using modern DNA markers. The markers developed in this study provide an important genomic resource for the sweetpotato community and contribute to the understanding of the genetic diversity present in the U.S. sweetpotato germplasm.
2. Understanding the ability of whiteflies to transmit plant viruses. The whitefly Bemisia tabaci is a major agricultural pest in the United States as well as around the world due to the damage caused by its feeding and because of the many types of viruses that it transmits to plants. ARS researchers at Charleston, South Carolina, along with other multi-disciplinary federal and state collaborators, recently reported on the first genomic sequence of a whitefly. Subsequently, the team conducted a gene analysis of the whitefly Bemisia tabaci on tomato infected with the Tomato yellow leaf curl virus (TYLCV) that identified whitefly genes that may be responsible in regulating the ability of this whitefly to acquire and transmit this virus. The findings contribute to the greater understanding of how TYLCV and related viruses affect the whitefly vector and provides a list of candidate genes that may be used in the development of genomics-based technologies for whitefly management.
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