WERA1007: Curtovirus Biology, Transmission, Ecology, and Management
(Multistate Research Coordinating Committee and Information Exchange Group)
WERA1007: Curtovirus Biology, Transmission, Ecology, and Management
Duration: 10/01/2016 to 09/30/2021
Statement of Issues and Justification
Curtoviruses are the most widespread geminiviruses in the United States. Curly top viruses, which are transmitted by the beet leafhopper, Circulifer (=Neoaliturus) tenellus (Baker), have caused significant problems to irrigated agriculture in the western USA, including California, New Mexico, Idaho, Arizona, Colorado, Texas, and Washington since 1899 (Carsner and Stahl, 1924). The viruses infect a broad host range from many plant families and the leafhopper vector also feeds and breeds on an extensive range of plant hosts (Bennett, 1971). The viruses cause economic damage to a wide variety of crops including tomato, pepper, dry beans, sugarbeet, melons, and leafy greens in the western U.S. The impact of the disease is dependent on crop and location.
Curly top has a long history of major impacts on California agriculture starting in 1919 affecting sugar beet and to a lesser extent beans and melons. The state began a monitoring/management program for the beet leafhopper in 1943 and has continually followed the virus vector through this funded program since that time. Curly top rarely impacted California processing tomatoes before the mid 1960’s, but as tomato production moved from directed seeded to transplants and from northern valleys to the Central Valley in the 1990’s, the risk for loss from BCTV greatly increased. In 2013, large leafhopper populations caused unprecedented losses in processing tomatoes in many fields of more 40%, resulting in the loss of about one million short tons of fruit.
While the precise monetary losses to the tomato industry are not known, the costs associated with the monitoring/spray program (BCTV Control Program) are available. Program income from grower assessments for fiscal years 2009/10-2013/14 was $1,544,051. Average annual Program expenses for the same fiscal years were $1,278,218. Since the outbreak in 2013, Program expenses have increased. For fiscal year 2014/15, which included the winter outbreak of 2015 when treatment and survey efforts greatly increased, expenditures were much higher than normal at $2,402,395, which included $905,000 in direct treatment costs (aerial spray contracts and pesticides).
During the 1920s and up to mid-1930s, curly top had the potential to eliminate the sugar beet crop in Idaho during years with large early beet leafhopper infestations. More recently, the beet industry in Idaho and Wyoming estimated that 70,000 of its 180,000 acres of sugarbeets were at high risk, another 70,000 acres were at medium risk and the remaining 40,000 were at low-low medium risk of curly top losses. The high risk acres of beets have been subject to 50-60% yield losses 2 out of 10 years. The medium risk beets are subject to 25-40% losses 1.5 out of 10 years. The low-medium risk beet acres are difficult to measure but likely 0-10% losses 1 out of 10 years. Using the 2015 yields of 38 Ton/acre and a $68 per ton beet payment and frequency of losses, estimated losses can be calculated at up to $1,290/acre for a 50% loss.
Currently, the Red River Valley, ID (approx. 51% of U.S. production) and sugar beet production in the western U.S. (approx. 35% of U.S. production) account for approximately 86% of the sugar beet production in the U.S. The total value of the U.S. sugar beet crop ranged in value from 1.4 to 2.4 billion dollars from 2012 to 2014. (Panella et al. 2014).
Curly top affects peppers and tomatoes in New Mexico. Losses to chile yields are variable, and have ranged from 50% in 2001 to 0.5% in 2012. In Washington, curly top is also quite variable in dry beans with losses of 50-75% in large acreages (500-1,000 acres) in certain years.
Curly top epidemiology in the western U.S. is dependent on area, climate, plant diversity and distribution, and cropping cycles (Bennett, 1971). Generally, the beet leafhoppers overwinters on weed hosts, acquires virus from the weeds, and migrates into agricultural areas to feed on (and infect) crops and weeds. In some areas of California, leafhoppers migrate back to the overwintering areas in the fall (Cook, 1967).
Curly top viruses are monopartite viruses in the family Geminiviridae, genus Curtovirus, which are characterized by a circular ssDNA genome within twin spherical particles. Molecular characterization of curtoviruses in sugarbeet in the 1980s, demonstrated that the viruses existed as three strains and variants of these strains (Stenger and McMahon, 1997), which were later determined to be distinct species. In New Mexico, we identified the curtoviruses Beet severe curly top virus (BSCTV) and Beet mild curly top virus (BMCTV), as well as several newly identified viruses, Pepper yellow dwarf virus (PeYDV), Pepper curly top virus (PeCTV), in chile and weed hosts and these were determined to be recombinants (Creamer et al, 2005; Lam et al, 2009). Curtoviruses were recently reclassified (Varsani et al., 2014) and currently almost all curtoviruses are now considered to be strains of beet curly top virus. As such, the curtoviruses listed above that we identified in New Mexico are now designated as BCTV-SvrPep, BCTV-Mld, BCTV-PeYD, and BCTV-PeCT, respectively.
Due to the ability of the virus to infect a large diversity of weeds and the ability of its insect vector to survive on a similarly large and diverse group of weeds, as well as migrate considerable distances, curtoviruses are endemic in the western U.S. Management of this viral pathogen and its leafhopper vector has proven difficult.
Use of a variety of insecticides with different active ingredients registered on a crop has been used in an attempt to control curly top. However, since the vector needs only a brief feeding interval in which to introduce the virus into a healthy plant, insecticides will not block virus transmission. Soil treatments with the systemic insecticide, imidicloprid, can reduce virus incidence on sugarbeets (Wang et al., 1999). Similarly, planting sugarbeet seed treated with Poncho Beta insecticide, leads to decreased levels on virus on sugarbeets (Strausbaugh et al., 2012). Sugarbeets are effective breeding hosts for the leafhopper. However, chile and tomato are not breeding hosts of the insect, like sugarbeets, and the leafhopper spends very little time on the plants (Hudson et al., 2010), so that even the use of systemic insecticides on chile and tomatoes have not proved efficacious in preventing curtovirus spread or infection. Even in crops such as sugarbeet, where insecticide use has been effective, the industry perceives their use as a temporary solution since the leafhoppers are expected to develop resistance in time.
California has long used an industry-funded pesticide spray program to control the leafhopper using organophosphate sprays to uncultivated foothill breeding areas (Clark, 1995), although that program has not always been effective. The BCTV Control Program sprayed about 24,700 acres in 2012, 52,330 acres in 2013, 49,000 acres in 2014, and 89,250 acres in 2015. The spray program utilized both ground and air spraying in the San Joaquin and Coastal Valleys of California based on sweep net counts in leafhopper breeding areas. An aggressive spray program was implemented in 2015 that combined spraying in winter, as well as spring, instead of sprays in spring and fall because of the early large sweep net counts in the breeding areas.
Knowledge of when and where to spray for most effective control of the leafhoppers in CA has also proven somewhat difficult. Timing of BLH outbreaks is highly variable and unpredictable because they are strongly dependent upon particular weather patterns. Historically they tended to happen about once every 6-10 years. With the CA drought the last 4-5 years, it has impacted the BLH so that surveys identify BLH much further north and east than historically observed.
Plant resistance has been or is being developed for impacted crops. While curly top resistant sugar beet cultivars have been developed, resistance is still only low to intermediate in the commercial sugar beet crop. However, all commercially grown chile pepper and tomato varieties are susceptible to the virus (Sedano et al., 2012). Cultural control in chile has relied primarily on overseeding the crop or removing infected plants. Biocontrol, including releasing imported egg parasitoids to control the leafhopper vector was evaluated in California. Predictive models of disease have been developed for specific areas, but do not eliminate the disease.
A sustainable management program is needed for this difficult pest/pathogen combination. Since leafhoppers migrate across state (and country) lines, management requires a coordinated effort between the different affected areas. Any approach to management requires an understanding of the genetic variability of the pathogen and vector among the different crop hosts of curtoviruses to be successful. Knowledge of the viral distribution within the region in wild and cultivated host plants, and the proximity of these virus reservoirs to commercial production fields is essential to reduce viral disease incidence. Since an integrated approach is necessary, significant progress in management of this viral disease will be possible only when individuals with different areas of expertise work together.
We propose the continuation of the WERA1007 group to address the problem. This committee will continue to meet to discuss, assess, and prioritize required research into curtovirus genetics, vector biology and genetics, weed ecology, and disease management. Individuals will share their best practices for management, which is the greatest benefit for active participation in the group, as rated by the current and past members. The committee will coordinate action plans to determine who will accomplish which aspects of the research, including who will work together to seek funding for the highest priority research. The group will also coordinate research to provide preliminary information needed to secure grant funding.
Related, Current, and Previous Work
There are no other related ongoing regional projects that address this topic. This project has been ongoing since 2004, first as a Western Region IPM working group and then as WERA 1007. Accomplishments of the group and of specific project members toward curly top management are listed below.
The largest impact of the WERA 1007 group has been development of sustainable collaborations among group members and sharing of information. For example, studies on strain emergence involve several laboratories in the group to varying degrees including the Wintermantel Lab, Gilbertson Lab members (LiFang Chen and Ozgur Batuman), Jennifer Willems at CDFA-CTCB, Oliver Neher and Greg Dean at Amalgamated Sugar (Idaho), and Carl Strausbaugh (Idaho). Studies on mixed infections have involved the Creamer lab, the Gilbertson Lab and the Wintermantel Lab. Studies on the factors effecting leafhopper transmission involve the Creamer lab and Carl Strausbaugh. Jennifer Willems has an ongoing collaborative project with Bob Gilbertson’s lab where she sends California field sample for strain identification as part of the larger curly top project, which has been active for the past few years.
Previous accomplishments in Idaho include the demonstration of the effectiveness in 2006 of clothianidin ([C(E)]-N-[(2-chloro-5-thiazolyl)methyl]-N’-methyl-N’’-nitroguanidine; Poncho, NipsIt), a neonicotinoid insecticide seed treatment, for control of BCTV in sugarbeet (Strausbaugh et al., 2006). The clothianidin seed treatment along with thiamethoxam (3-[(2-chloro-5-thiazolyl) methyl]tetrahydro-5-methyl-N-nitro-4H-1,3,5-oxadiazin-4-imine, marketed as Cruiser) has become a valuable supplement to host resistance through control of the beet leafhopper vector (Strausbaugh et al., 2006, 2012). The treatment has provided a yield increase of 17% and the seed treatments helped make the transition to the use of glyphosate-resistant cultivars possible in sugar beet in the semi-arid western U.S. Some production areas most heavily influenced by curly top such as the Western Sugar production area (Colorado, Nebraska, and Wyoming) require the neonicotinoid seed treatments on all acres. The seed treatments provide very good general pest control as well and are now used on approximately 60 to 70% of the acreage in the Red River Valley.
Previous accomplishments in Washington include the establishment of a beet leafhopper monitoring network that spans the entire Columbia Basin irrigated production region (700,000 acres). This provides growers (650 current subscribers) with an early warning, through pest alerts, about potential beet leafhopper-related problems (Wohleb, unpublished).
The biggest accomplishments for the California CurlyTop Program are improved communication to growers to alert them of potential devastation (throughout the year) by testing numerous samples of beet leafhoppers and host plants all year long and giving them info on pest pressure and virus pressure in real time, and increased educational outreach to growers, PCA’s, UC extension farm advisors, to teach them how to identify a beet leafhopper versus other non-target leafhoppers. Communication and education has had a tremendous positive impact for the tomato industry in CA. In 2015, the Program’s survey efforts picked up on the developing threat earlier, and the Program worked vigorously with the affected grower community to obtain permission to treat more properties. As a result, about twice as much of the prime leafhopper breeding ground was treated.
The biggest impact of the California USDA effort involves work with two regions and two industry groups: California and Idaho for tomato and sugarbeet, respectively. In recent years the most significant work is on characterization of emerging strains and determining their competitiveness against traditional forms of BCTV, and efforts toward commercialization of a biotech-based strategy for control of BCTV. The studies on strain emergence and dominance is important for the development and hopeful commercialization of methods of BCTV control using biotechnology. The former studies on strain emergence allow us to monitor the BCTV strains that should be targeted, and be certain as we work with commercial partners that our materials will be effective against the common variants of the virus in the field.
Assess the current status of curly top and set priorities for integrated research on curly top disease.
Organize research on curtovirus genetics, genetics and biology of populations of the beet leafhopper vector, and the role of weed hosts in curly top in the western US.
Organize research to examine integrated strategies for managing curly top.
Develop best practices strategies for management of curly top.
Provide opportunities and resources for formal and informal education on curly top disease, virus/insect/plant ecology and interactions, and management.
Procedures and Activities
- Assess the current status of curly top and set priorities for integrated research on curly top disease.
Individuals working on different aspects of curly top in the western US come together annually to improve communication among different disciplines. Those that have met with the group in the past include virologists, weed scientists, entomologists, plant breeders, and extension scientists from New Mexico, Arizona, California, Colorado, Idaho, and Washington. The group has been approximately 25% each AES researchers, CES scientists, USDA scientists, and growers/industry representatives. Regional stakeholders with a commitment to the working group and its research include the New Mexico Chile Commission, New Mexico Chile Association, the New Mexico Farmers Marketing Association, the California Beet Sugar Association, California Tomato Research Institute (Fresno, CA), the Beet Sugar Development Foundation (Kimberly, ID), Curry Seed Co (Pearce, AZ), and the California Department of Food and Agriculture Curly Top Control Program, the Amalgamated Sugar Co. (Boise, ID). The coordinating committee will meet yearly to discuss the status of curly top in the western US and present the latest developments in curly top research. At the annual meetings, the group will also discuss gaps in the knowledge of the disease and set priorities for research.
The top priorities for the group for the upcoming 5 years are to monitor the virus strains in each location and improve the disease prediction model, including within season spread in all locations and to test and refine the model in each major cropping area that is affected by the disease.
- Organize research on curtovirus genetics, genetics and biology of populations of the beet leafhopper vector, and the role of weed hosts in curly top in the western US.
The ability to manage a virus disease requires an understanding of which virus is causing the disease. Committee members have demonstrated that viral genetic diversity is present in a variety of plant hosts and that new curtoviruses appear regularly. Within cooperating western states, there are efforts underway to study the virus incidence and diversity, for example in sugarbeets in Idaho and Wyoming, in peppers in New Mexico, and in tomatoes in California. These reports show that genetic diversity within curtoviruses is very high and will require a concerted effort to identify the tremendous virus diversity throughout the western US. The group will continue to share information on viral genetic diversity and encourage attempts to gain funding to study virus genetic diversity throughout the region by sequencing the genome of curtovirus isolates collected from different crops, weeds, and states. The compilation of this information is also essential for effective development of technology-based management systems.
While much research has been done to determine the leafhopper host range, the biology and ecology of the leafhopper in specific locations differs with cropping patterns and weed populations. Research to assess how the leafhoppers are overwintering has been done for some states, but not others and similarly, leafhopper flight patterns have been studied in some areas and not others. These research questions need to be expanded to additional areas in the western US.
Preliminary information suggests that beet leafhopper populations from different states vary genetically according to analysis of the mitochondrial DNA. The group will encourage leafhopper collections from diverse populations to assess the vector leafhopper genetic variability. Leafhopper populations from the different breeding areas may be geographically isolated or interbreeding populations. If they are reproductively isolated, then there may be important differences in adaptation to local host plants, host range and preferences, and virus strain-vector specificity. If they are interbreeding populations that exhibit a similar phenotype, then it may be possible to devise broadly based disease control strategies. One member of the committee has begun to obtain the nucleic acid sequence of the beet leafhopper. We hope to use this resource to analyze other genes from the leafhopper that may be useful for differentiating between populations.
There are many reports on the weeds hosts of BCTV and its leafhopper vector. However many of these reports are historical, and the weed populations have changed significantly in the last 50 to 80 years since the reports were published. Virus incidence in weeds has been reported for California and New Mexico. Similarly overwintering and key weed hosts have been studied in the two states. California is now beginning to use the methods developed in New Mexico to study the leafhopper population movements. The group will also encourage combining data for the leafhoppers and viruses into GIS mapping studies for each primary vector breeding area. Data on the movement of leafhoppers from weeds to crop plants will be incorporated into predictive models.
- Organize research to examine integrated strategies for managing curly top.
Curly top has proved very difficult to control. Project members have studied a variety of methods to manage the disease including biocontrol of the leafhopper vector, impact of insecticide sprays, use of anti-transpirants to deter leafhopper feeding, effectiveness of trap crops, row covers to deter insects, identification of plant resistance to virus and leafhoppers, engineered resistance, and predictive modeling to determine the likelihood of curly top in the next growing season and timing of leafhopper flights. All areas where curly top occurs are very interested in adapting curly top prediction models for their locations, specific weed populations, and environmental conditions. The group will continue to coordinate management tests in more than one location. The group will continue to help distribute management tools and products, such as resistant varieties and predictive models. Management tools developed in one crop can be easily transferred into a new crop. For example, previous work by the Creamer lab helped the CA Curly top Program develop a trapping method to help growers and sparked a research project for testing the effectiveness of kaolin clay on organically grown tomatoes in CA. This has led to a test plot last fall and again this spring. In addition, we anticipate that information about the genetics of the virus and vector may lead to novel management strategies applicable to many locations. An example of a request from the tomato industry for help in management is for a BCTV rapid detection kit for in field detection. This sort of tool could be used by growers in a variety of crops impacted by curly top.
- Develop best practices strategies for management of curly top.
The group has accumulated much information on various aspects of curly top and has actively disseminated the information as it has been generated. Much of the information has been disseminated through peer-reviewed publications and at grower meetings and field days; however, general practical information on the disease and its management is not available in a usable format in a single location. Often, the information that is available is specific to a particular crop or location. The extensive industry interaction within the group has made the gaps in information availability more obvious. The committee, with directed input from the industry members, will develop best practices management recommendations, both general and specific, for each crop impacted by curly top.
- Provide opportunities and resources for formal and informal education on curly top disease, virus/insect/plant ecology and interactions, and management.
The group will develop a website to house information on facts about curly top and the best practices recommendations developed by the group. Just as important, the group plans to add new information as it becomes available, keeping the information current. For example, one member of the group, William Wintermantel, has produced a webinar on curly top that could be added to the site.
The website will also be a repository for K-12 teaching modules related to curly top, its vector, weed hosts, and monitoring and predicting disease. Several modules have already been developed for middle school students for working with leafhoppers; additional modules will be developed dealing with ecology and epidemiology of diseases. Other modules will be added as needed.
The meetings of the group will continue to provide an opportunity for undergraduates and graduate students working on curly top to present their research and to meet industry representatives and established researchers. The importance of these meetings for students has not been acknowledged, but the small group format has generally provided an ideal opportunity for graduate students to present their research, with an average of two graduate student presentations each year.
The implementation of this proposal will result in continued annual meetings of the WERA 1007. At meetings, research progress reports will be presented, graduate students will present their research, and priorities for needed research into curly top will be identified. An annual action plan will be developed to determine which components of the group will accomplish which aspects of the needed work. The group will decide who will work together to seek funding for the top priority research. It is expected that preliminary research will be coordinated to obtain information needed to secure grant funding. A transcript of all meeting discussions will be made available to all committee members and interested individuals. Best practices management recommendations, both general and specific by crop will be developed. K-12 modules addressing aspects of curly top will be developed. A website will be established to house the information.
Expected Outcomes and Impacts
- Research will be coordinated at a regional level (among researchers at different locations), and become more interdisciplinary (among researchers from different disciplines).
- Management strategies and products, such as resistant plant varieties and predictive models will be extended to new locations.
- The agricultural industry will be able to influence the direction of the research.
- Information about curly top will be more widely disseminated, bringing attention to the disease and its control.
Projected ParticipationView Appendix E: Participation
All participants will be encouraged to disseminate information discussed at the meetings through grower and commodity board meetings as well as field days, pesticide applicator training, press releases, and extension and grower publications. Growers, crop consultants, processors, and extension personnel members of the group can help disseminate the information to their constituents and clientele. Transcripts of all meetings will be made available to all interested individuals upon request. Since the clientele has a large Hispanic base in New Mexico, Arizona, and Texas, and growers often conduct business in the US and Mexico, transcripts will also be translated into Spanish. To address the needs of smaller growers such as Native Americans or organic producers, relevant information will be distributed to representatives of these groups, such as the Federally Recognized Tribes Extension Program (FRTEP), who will also be encouraged to participate more actively in the coordinating committee. A website will be developed to allow broad dissemination of information to clientele and the public not served through the extension meetings and publications.
K-12 teaching modules related to curly top, its vector, weed hosts and monitoring and predicting disease will be developed. Several modules have already been developed for middle school students for working with leafhoppers and additional modules will be developed dealing with ecology and epidemiology of diseases. Participants will be encourage to develop educational modules based on current or past K-12 teaching and outreach related to curly top.
An Executive Committee made up of the chair, past chair, local arrangements chair, and secretary will serve as the governing board. The committee will also help plan meetings and write relevant reports. The Chair position will rotate among members annually and will transition each year at the annual meeting. Local arrangements chair for annual meetings will also be rotated among participants. Rebecca Creamer will initially serve as the secretary, keeping the email listing of participants, and maintaining all pertinent documentation.
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Carsner, E. and Stahl, C. F. 1924. Studies on curly-top disease of the sugar beet. J. Agr. Res. 28:297-320.
Clark, R. A. 1995. Environmental assessment of curly top virus control in California: 1991-1995. Cal. Dept. Food and Agr. Sacramento, CA
Cook, W. C. 1967. Life history, host plants, and migrations of the beet leafhopper in the western United States. U.S.D.A. Tech. Bull. 1365. 122 p.
Creamer, R., Hubble, H., and A. Lewis. 2005. Curtovirus infection of chile pepper in New Mexico. Plant Disease 89:480-486.
Hudson, A., Richman, D. B., Escobar, I., and Creamer, R. 2010. Comparison of the feeding behavior and genetics of beet leafhopper (Circulifer tenellus, Baker) populations from California and New Mexico. Southwestern Entomologist 35:241-250.
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