Project : USDA ARS

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Research Project: Sustainable Approaches for Pest Management in Vegetable Crops

2020 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 on whiteflies on vegetable crops was continued. Data on the resistance offered in wild relatives of watermelon were collected, and adult whiteflies exhibited negative behavior to odors of certain extracts. Genetic populations of whitefly-resistance watermelon were developed. A study on alternative reproductive host plants of the global whitefly pest Bemisia tabaci was conducted to determine if the plants could possibly serve as a bridge host through mild winters, as is common in the southeastern United States. Two common plant species, Brassica carinata winter cover crops that is related to collard plants) and wax myrtle have green foliage during the winter season. They both served as egg-laying host plants for this whitefly. Also, the data showed that carinata (is a good host, but wax myrtle was found not to be able to support whiteflies through the developmental period (both in the laboratory and in nature). Although chemical control is the last resort in managing whiteflies, a study was initiated to evaluate the efficacy of biopesticides in managing whiteflies, and the effect of the pesticides on natural enemies of whiteflies. Six biopesticides resulted in similar performance and support that they may offer a better option than traditional pesticides. In collaboration with the University of Georgia, whitefly populations survived the winter in field cages in ongoing research on impact of climate on whitefly populations. The collaborative research also assessed the efficacy of selected biopesticides in field trails against whiteflies; the study included several types of insecticides (systemic, foliar, and biorational) in several vegetable crops. The pesticide research may provide information on potential new active ingredients, new formulations, efficacy/resistance of insecticides for whitefly management, and fit for integrated pest management schemes. In additional collaborative research with the University of Georgia, laboratory experiments were conducted on the virulence of a new strain of a fungus named "GA17" as compared with a commercially available fungal strains against whiteflies and other insect pests. The investigation into the potential to use this fugus as a control agent is ongoing. Other collaborative research was done with the University of Georgia on the role of planting date and temperature on the disease incidence and severity of whitefly-transmitted viruses, and plant resistance to whiteflies and whitefly-transmitted viruses. In collaborative research with University of Georgia and Fort Valley State University, research was done on the distribution and density of whitefly populations in the southeast. In collaborative research with the University of Georgia, research was conducted to develop genetic technology as a method to disrupt whitefly reproduction and thus limit population growth. Germplasm development of sweetpotato continued, with over 20,000 seeds harvested from two open-pollinated breeding nurseries. A mapping population for segregation of Meloidogyne enterolobii was established and over 500 seeds were harvested. Field evaluation of over 5,000 seedlings and 2% of these were selected for a combination of either bunch type growth habit and/or insect resistance. Four advanced clones were sent to the National Clean Plant Network for virus elimination. Over 150 intermediate, advanced, and regional sweetpotato selections were maintained in field plots and tissue culture. A single breeding nursery was established for continued development of new sweetpotato germplasm and a single bi-parental mapping population was developed that will segregate for multiple traits. Research to develop molecular markers to characterize the genetic diversity of sweetpotato weevils (Cylas formicarius elegantulus) from Georgia, Hawaii, and South Carolina was completed. A total of 5,000 sweetpotato seedlings were evaluated and 1.5% of the seedlings were jointly selected between USDA and North Carolina State University for further evaluation by both institutions. Research was continued on sweetpotato cultural practices (including tillage and irrigation practices, and weed interference) on soil arthropod abundance and pest damage to storage roots. Through financial support from the potato industry and ARS, 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 (i.e., click beetles) – progress was made on several species in this study. Field trials were initiated with a click beetle sex pheromone to improve trapping technology, and to characterize the pest’s seasonal phenology. Studies were planned to continue the development of improved detection and monitoring methodologies of vegetable pests 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 reproductive behavior. Studies to evaluate the effect of cover crops on soil arthropod species composition and abundance during transition from conventional to organic production practices were completed. A team of ARS researchers and other multi-disciplinary state collaborators, sequenced the genome of a whitefly for the first time. The genomic sequence accomplishment was selected for showcase in the publication BNC Biology, and has received much interest by domestic and international scientific communities. Research was done which enhanced the understanding of the ability of whiteflies to transmit plant viruses, including gene analysis of the whitefly Bemisia tabaci on tomato infected with Tomato yellow leaf curl virus (TYLCV) that identified whitefly genes that may be responsible in regulating the ability of whitefly to acquire and transmit this virus. Methods were developed for DNA isolation from trapped sweetpotato weevil and molecular markers developed for characterization of genetic diversity. Microsatellite markers were developed and characterized and we found low genetic diversity which is indicative of an introduced pest. Sweet potato leaf curl virus (SPLCV) was determined not to be transmitted through seeds based on extensive assays. These results support that seeds can be shared among breeding programs without concern of spreading the SPLCV through the seeds. The whitefly-transmitted Cucurbit leaf crumple virus, a problem in cucurbit crops, was found to spread to South Carolina. These results have implications on the management of whiteflies and associated viruses in the southeastern United States. Three single-nozzle operator-carried backpack spray applicators were assessed for whitefly control on summer squash with five biorational and conventional insecticides. All insecticides suppressed the whiteflies; mortality ranged from 73% to 95%. A study with a neem-based biorational insecticide on whiteflies on collards indicated repellency to the adults, and egg laying and survival of the immatures were reduced. In a collaborative study (with an Egyptian researcher), several biorational insecticides suppressed whitefly populations in vegetable seedlings and delayed whitefly-transmitted viruses. Research was done which 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. Field trials using this pheromone were initiated to improve trapping technology, and to characterize the pest’s seasonal phenology in the mid-Atlantic states. Research was conducted on the effects of sweetpotato cultural practices (including plasticulture mulch and irrigation practices) on soil arthropod abundance and pest damage to storage roots. Studies continued to evaluate the effect of cover crops on soil arthropod species composition and abundance during transition from conventional to organic production practices. Research was continued on the assessment of climate and other factors affecting whiteflies and a whitefly predator (D. catalinae). A genetic (transcriptome) analysis was conducted on the whitefly B. tabaci on tomato infected with the TYLCV, and whitefly genes were identified that may be responsible in regulating the ability of the whitefly to acquire and transmit this virus. Screening of cucumber for resistance to pickleworm was done, with over 700 plant introductions planted into field plots to identify sources of resistance. Screening of over 100 sweetpotato plant introductions (PIs) from the USDA, ARS Sweetpotato Germplasm Collection for identification of sources of resistance to the Guava root-knot nematode [GRKN (Meloidogyne enterolobii)] was completed. In total, 20 sweetpotato PIs have been identified with a high level of resistance to isolates from North Carolina and South Carolina. Resistant PIs have been incorporated into the breeding nurseries for introgression of resistance. The population structure and genetic diversity of over 400 sweetpotato types from the USDA, ARS germplasm collections were 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. Results of phenotyping of root and leaf traits were published for over 700 PIs from the USDA, ARS Sweetpotato Germplasm Collection. This information provides important data that can be used for linking phenotype with genotype in sweetpotato. Promising advanced selections of sweetpotato clones (10) were identified that are insect resistant and/or competitive with weed pressure.

Accomplishments
1. Sweetpotato weevil genetic diversity. The sweetpotato weevil is a serious insect pest of sweetpotato in the field and storage and occurs in over 50 countries. Information is lacking regarding the genetic diversity and population structure of the sweetpotato weevil within the U.S. While resistant sweetpotato genotypes have been identified from breeding efforts worldwide, it is unclear whether this variability is attributed to environmental effects, insect distribution or genetic differences, or true host resistance. New knowledge of the genetic diversity and population structure of sweetpotato weevils can provide information for solving the genetic effect of the sweetpotato weevil on host resistance. ARS researchers In Charleston, South Carolina, in collaboration with researchers at the University of Tennessee, developed molecular markers that were used to characterize population structure and genetic diversity within the sweetpotato weevil. Our study demonstrated conclusively that diversity in three populations of sweetpotato weevil is low, which is consistent for an introduced pest. Knowledge on the population structure and genetic diversity is critical information for developing durable host resistance and providing important insights for developing management strategies. ARS researchers in Charleston, South Carolina, demonstrated the suitability of the microsatellite loci for conducting range-wide population studies for this important sweetpotato pest. These results are important to researchers and the sweetpotato industry.

2. New host plant for the sweetpotato whitefly. ARS researchers in Charleston, South Carolina, studied two plant species, Brassica carinata (carinata or Ethiopian mustard) and Myrica cerifera (southern wax myrtle) as potential overwintering host plants for the sweetpotato whitefly because of their presence with green foliage in the southeastern United States agricultural community from the fall through spring season. This whitefly survives mild winters in the southeastern United States. This study demonstrated for the first time that carinata is a host plant for this whitefly; the study also demonstrated that this is a plant that is favorable for this pest to build up its population. However, the southern wax myrtle only served as a host for egg laying, while the insects did not fully develop. Winter populations of this whitefly may be suppressed in the field, but any host plant species such as carinata, that are present during the mild winter may help support a buildup of whiteflies during the spring. The results of this study have implications to growers, pest management professionals, and researchers.

3. Discovery of pest sex pheromone. Sweetpotato is subjected to numerous pests, many of which cause damage to the underground portion of the plant. These pests spend most of their life cycle underground, coming to the surface only to disperse, mate, and lay eggs. Their subterranean life history makes identification, detection, monitoring, and management of the pests a challenging endeavor. A better understanding of the reproductive biology of the adult (i.e., aboveground) stage of these pests may provide novel options for pest management. However, little is known about how these insects reproduce. ARS researchers in Charleston, South Carolina, in collaboration with university researchers in California and South Dakota, isolated, identified, synthesized and evaluated the sex pheromone of a pestiferous click beetle important to crops in much of the eastern U.S. This constitutes the first identification of a pestiferous click beetle pheromone in North America. Results indicate that the pheromone is species-specific and highly attractive. Discovery of this pheromone will lead to further studies to develop biologically-based management strategies for sweetpotato pests, such as integration of the pheromone with insect pathogens, or mating disruption, and this will benefit the sweetpotato industry.

U.S. DEPARTMENT OF AGRICULTURE

Vegetable Research: Charleston, SC