Project : USDA ARS

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Research Project: Sustainable Approaches for Pest Management in Vegetable Crops

2019 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. 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.

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
Research was continued on sweetpotato cultural practices (including plasticulture mulch, irrigation practices, and weed interference) on soil arthropod abundance and pest damage to storage roots. With financial support from the sweetpotato industry, several studies at regional, national, and international levels, were initiated to isolate, identify, and evaluate insect-produced chemicals that affect the reproductive behavior of insect pests of sweetpotato (e.g., click beetles). Field trials with click beetle sex pheromone were evaluated on dose-response rates and release techniques. Work was initiated to evaluate the effect of entomopathogens on adult click beetles and their larval stage (wireworms). Research was done on the development of improved detection and monitoring methodologies of vegetable pests by incorporation of light cues with sex pheromones. Insect colonies of several species were established from field-collected insects; these 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. Sweetpotato germplasm development was continued with over 30,000 seeds harvested from two open-pollinated breeding nurseries. A total of 9,000 seedlings were evaluated in the field and 5,000 of these were from populations used for selection of bunch type growth habit. From the 5,000 seedlings, 95 sweetpotato clones were selected and are being further evaluated for tolerance to weed competition. Eighteen sweetpotato clones have been selected from 2016-2018 for bunch type growth habit and insect resistance and are being evaluated in field experiments to identify the most competitive against weed pressure. Over 150 intermediate, advanced, and regional sweetpotato selections were evaluated in field plots for insect resistance and for other important horticultural traits. Two breeding nurseries were established for continued development of new sweetpotato germplasm and a single bi-parental mapping population was developed. Screening of 83 sweetpotato plant introductions (PIs) from the USDA, ARS Sweetpotato Germplasm Collection was continued for identification of sources of resistance to the Guava root-knot nematode [GRKN (Meloidogyne enterolobii)]. In total, 20 sweetpotato PIs have been identified with a high level of resistance to a GRKN isolate from South Carolina. GRKN resistant PIs have been incorporated into the breeding nurseries for introgression of resistance. Research was initiated to develop a protocol for isolation of high quality DNA from sweetpotato weevils [Cylas formicarius elegantulus (Summers)] collected from pheromone-baited aerial funnel traps. Tolerance to the whitefly-transmitted Sweet Potato Leaf Curl Virus (SPLCV) is being evaluated in 25 sweetpotato plant introductions that have been released by the U.S. Vegetable Laboratory. Research continued on whitefly genomics, biology, management, and whitefly virus transmission. A 5-year agreement was established with the University of Georgia to research solutions to the problems from the annual whitefly and whitefly-viruses in Georgia and the other southeastern states. Through financial support from the pickle industry, screening of cucumber for resistance to pickleworm [Diaphania nitidalis (Stol) 1781] continued, with nearly 200 plant introductions planted into field plots to identify new sources of resistance.

Accomplishments
1. Method for DNA isolation from trapped sweetpotato weevil. The sweetpotato weevil is distributed throughout tropical and subtropical regions of the world and is a serious pest of sweetpotato in the field and in storage facilities, and it is now found throughout the coastal plains of the southeastern United States. Sampling for this pest is important, but information is lacking regarding the genetic diversity of sweetpotato weevil in the United States; however, an important first step toward assessing this diversity is the development of robust methods for isolation of genomic DNA from trapped specimens from collection samples. USDA-ARS researchers in Charleston, South Carolina, discovered that under summer conditions typical of sweetpotato production, it is critical to collect weevil specimens from traps at an optimum interval of one week. Furthermore, this work indicates that either 70% ethanol or storage at -20°C can be used to preserve samples that are no older than 7 days without a loss in the quality of the genetic material. Our findings allow for scheduling of sampling at reasonable times without the need for any special materials, and allows individuals without special training to collect sweetpotato weevil specimens for genetic studies.

2. Wild type watermelon with resistance against whiteflies. Whiteflies are a major problem in watermelon production, but traditional breeding approaches can be used to improve this crop against pests and diseases. A team of USDA-ARS scientists in Charleston, South Carolina, Griffin, Georgia, and Oxford, Mississippi, have identified and made the first report of pest resistance in the wild desert plant Citrullus ecirrhosus which is a relative of cultivated watermelon. The performance of whiteflies is low when whiteflies attempt to colonize this plant. The team also demonstrated that traditional breeding methods can be used to cross this wild plant with cultivated watermelon and produce viable seeds. This wild plant offers a natural source for breeding for plant resistance against whiteflies in cultivated watermelon.

3. Cucurbit leaf crumple virus spreads to South Carolina. The Cucurbit leaf crumple virus is a problem in cucurbit crops; it has previously been reported in Florida and Georgia as well as in the southwest (California, Arizona and Texas). This plant virus is transmitted by whiteflies and can result in reduced yield, and stunted or dead plants. A team of USDA-ARS, university, and industry investigators in South Carolina and Florida reported on the first detection of this virus in South Carolina after finding it infecting fields of watermelon, cantaloupe and squash. These results have implications on the management of whiteflies and associated viruses in the southeastern United States.

4. Protection of sweetpotato. Sweetpotato is attacked by numerous pests, many of which cause damage to the underground portion of the plant. These pests spend most of their life cycle underground, and they come to the surface only to disperse, mate, and lay eggs; this makes identification, detection, monitoring, and management of these pests challenging. A better understanding of the reproductive biology of the adult stage (which occurs aboveground) of these pests may provide novel options for pest management. USDA-ARS scientists in Charleston, South Carolina, in collaboration with university researchers in California and South Dakota, isolated, identified, synthesized and evaluated the sex pheromone of a click beetle pest that damages crops in much of the eastern U.S. This is the first identification of a pestiferous click beetle pheromone in North America, and we found this pheromone to be species-specific and highly attractive. The discovery of this pheromone will lead to further research to develop biologically-based management strategies for sweetpotato pests, such as integration of the pheromone with insect pathogens, or mating disruption.

U.S. DEPARTMENT OF AGRICULTURE

Vegetable Research: Charleston, SC