Location: Vegetable Research
2017 Annual Report
Objectives
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 [NP304, Component 3, Problem Statement 3A1].
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.
Approach
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.
Progress Report
New sweetpotato germplasm was developed through open pollinated crosses from three breeding blocks. Over 40,000 seeds were harvested and 10,000 seeds were offered to collaborators. DNA from about half of the USDA-ARS Sweetpotato Germplasm collection was sent for genotyping by sequencing. Selection of improved sweetpotato germplasm continued with over 10,000 1st year seedlings and over 150 2nd year seedlings, intermediate, advanced, and regional clones; these were evaluated in replicated field plots for insect resistance and other important horticultural traits. Two breeding blocks were established and over 50 sweetpotato clones were maintained in unreplicated field plots. 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 were further evaluated. Research was started on sweetpotato cultural practices (including plasticulture mulch and irrigation) on soil arthropod abundance and pest damage to storage roots. Selections (n=20) from 2016 for compact plant habit were established into replicated field trials to further select for the most competitive clones against weed pressure. Over 2,300 seedlings were evaluated and 77 selections were made that exhibited compact growth and reduced light penetration through the canopy. These selections will be evaluated further in weed competition studies. Over 70 sweetpotato lines were selected and obtained from the USDA-ARS Sweetpotato Germplasm collection and the National Clean Plant Network and they are maintained in vitro for studies on tolerance to Sweet potato leaf curl virus (SPLCV). Leaf tissue from over 12,000 sweetpotato seedlings were tested for seed transmission of SPLCV. All samples tested negative for SPLCV indicating 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 continued on the assessment of climate and other factors affecting whiteflies and a whitefly predator (Delphastus catalinae) that might be used as a biocontrol agent.
Over 500 plant introductions of cucumber were planted into field plots to identify sources of resistance to pickleworm. Field and laboratory studies were continued to isolate, identify, and characterize chemicals that affect the behavior of insect pests of sweetpotato (i.e., click beetles). 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 were also started to evaluate the effect of cover crops on soil arthropod species composition and abundance during transition from conventional to organic production practices.
Accomplishments
1. Understanding whitefly genetics enhanced by the full genome sequence. A team of USDA-ARS scientists in Charleston, South Carolina, along with other multi-disciplinary federal and state collaborators, sequenced the genome of a whitefly for the first time. There are over 1,500 types of whiteflies, but Bemisia tabaci feeds on over 1,000 types of plants and is by far the most problematic whitefly in crops in the U.S. and on a global scale. Damage from feeding and from infections by plant viruses that are transmitted by this whitefly cause crop losses in billions of U.S. dollars yearly, and contribute to famine in some areas of Africa. The genomic sequence accomplishment was selected for showcase in the publication BNC Biology, and has received much interest by domestic and international scientific communities. The research is a major advancement for scientists working to understand whitefly biology and to develop solutions for growers to control whiteflies as well as the control of whitefly-transmitted plant viruses in crops.
2. Sweetpotato leaf curl virus not transmitted through seeds. Whiteflies are well-known to transmit the Sweet potato leaf curl virus which is an important plant virus that negatively affects sweetpotato production. Sharing seeds among sweetpotato breeding programs facilitates cultivar development, but this cooperation could be stopped if the seed could result in virus-infected plants. Leaf tissue from over 12,000 sweetpotato seedlings from seeds originated from infected plants all tested negative for the sweetpotato leaf curl virus by USDA, ARS scientists in Charleston, South Carolina; this indicates a lack of seed transmission by this virus. These results support that seeds can be shared among breeding programs without concern of spreading the Sweet potato leaf curl virus through the seeds.
Review Publications
Daniels, D.A., Lord, R.J., Nix, K.A., Wadl, P.A., Vito, L.M., Wiggins, Gregory, J., Windham, M.T., Ownley, B.H., Lambdin, P.L., Grant, J.F., Merton, P., Hadziabdic, D. 2016. Thousand cankers disease complex: a forest health issue across the U.S.. Forests. 7:260. doi:10.3390/f71102260.
Chen, W., Hasegawa, D.K., Kaur, N., Kliot, A., Pinheiro, P.V., Luan, J., Stensmyr, M.C., Zheng, Y., Liu, W., Sun, H., Xu, Y., Luo, Y., Kruse, A., Yang, X., Kontsedalov, S., Lebedev, G., Fisher, T., Nelson, D.R., Hunter, W.B., Brown, J.K., Jander, G., Cilia, M., Douglas, A.E., Ghanim, M., Simmons, A.M., Wintermantel, W.M., Ling, K., Fei, Z. 2016. The draft genome of whitefly Bemisia tabaci MEAM1, a global crop pest, provides novel insights into virus transmission, host adaptation, and insecticide resistance. BMC Biology. 14:110 doi:10.1186/s12915-016-0321-Y.
Krzyzaniak, D., Simmons, A.M., Shepard, M.B. 2016. Effects of a foliar neem formulation on colonization and mortality of whiteflies (Hemiptera: Aleyrodidae) on collard plants. Agricultural Sciences. 7:771-782. doi:10.4236/as.2016.711071.
Levi, A., Simmons, A.M., Massey, L.M., Coffey, J., Wechter, W.P., Jarret, R.L., Tadmor, Y., Nimmakayala, P., Reddy, U. 2017. Genetic diversity in the desert watermelon Citrullus colocynthis and its relationship with Citrullus species as determined by high-frequency oligonucleotides-targeting active gene markers. Journal of the American Society for Horticultural Science. 142(1):47–56. doi:10.21273/JASHS03834-16.
Li, Y., Harris-Shultz, K.R., Wang, H., Wadl, P.A., Ji, P. 2017. Population structure and genetic diversity of Phytophthora nicotianae from tobacco in Georgia. Plant Disease. 101:1113-1118.
Saadat, D., Seraj, A., Goldansav, S., Williams III, L.H. 2016. Factors affecting reproductive success and life history parameters of Bracon hebetor Say (Hymenoptera: Braconidae) from three host-associated populations. Biological Control. 96:86-92.
Amissah, J.N., Wadl, P., Hadziabdic, D., Boggess, S., Trigiano, R. 2016. Characterization of thirteen microsatellite loci from the Ghanian antimalarial plant Cryptolepis sanguinolenta. Journal of Medicinal Plants Research. 10(14):183-187.
Edwards, T.P., Trigiano, R.N., Wadl, P.A., Ownley, B.H., Hadziabdic, D. 2016. First report of Alternaria alternata causing leaf spot on Ruth's golden aster (Pityopsis ruthii) in Tennessee. Plant Disease. 101:383. doi:10.1094/PDIS-08-16-1214-PDN.
Edwards, T.P., Trigiano, R.N., Wadl, P.A., Ownley, B.H., Hadziabdic, D. 2016. First report of Alternaria alternata causing leaf spot on the whorled sunflower (Heilianthus verticillatus) in the southeast United States. Plant Disease. 101:632. doi:10.1094/PDIS-08-16-1216-PDN.
