A New Threat to Outdoor Hemp and Cannabis: Beet Curly Top Virus
by Katrina Tso
What is Beet Curly Top Virus?
Beet Curly Top Virus, or BCTV, is a wide-spread plant disease across the western United States and beyond and is spread exclusively through the feeding by the beet leafhopper. It is known to infect over 300 different plant species including sugar beets, spinach, melons, peppers, tomatoes1, and most recently, hemp2.
Confirmed using genetic analysis in a hemp sample from the North Fork Valley of Colorado in 20182, BCTV is likely widespread in the crop given the disease’s rapid spread throughout the infected hemp field and the confirmed geographic distribution of the disease in other crops1. The threat this disease poses to hemp is great considering the complex biology of the disease and the occurrence of up to 100% crop loss in individual fields across numerous commodities3. California in 2013 reportedly cost economic losses to farmers of more than $100 million1.
Known global distribution of BCTV1
What are the symptoms of BCTV in hemp?
In Colorado, BCTV was observed to infect numerous varieties across all different growth stages. When a hemp plant first becomes infected with BCTV, leaves begin to fade to a pale green or yellow green starting at the base of the leaf and expanding towards the tip, often resulting in a mosaic pattern on the leaves. The majority of the plant will be affected by this pattern of discoloration within approximately 10 days. Late stage disease can cause a number of symptoms in various combinations, including newly developing leaves to be pale in color, narrower leaf development, leaf curling, severe stunting, and drastic reduction in yield2. In some occasions, hemp plants have been observed to be bi-symptomatic, with only part of the plant showing signs of disease4.
Early mosaic pattern symptom of BCTV in hemp leaf. Photo from Nachappa & Chiginsky, 2019.
Twisting of new growth from BCTV infection. Photos from Nachappa & Chiginsky, 2019.
Early mosaic pattern symptom of BCTV in hemp plant. Photo courtesy of Giladi et al., 2020.
Yellowing and stunting of late stage disease. Photos from Nachappa and Chiginsky, 2019.
Mid-stage mosaic pattern and leaf curling symptoms of BCTV in hemp plant. Photo courtesy of Giladi et al., 2020.
The adult beet leafhopper measures only 0.12 inches long and is most active in high temperatures. Photos by G. Oldfield, USDA, Bugwood.org.
Late-stage BCTV in hemp plant, with severe leaf curling, stunting, and chlorosis. Photo courtesy of Giladi et al., 2020.
The beet leafhopper is often greenish, yellow, tan or olive in color, and may have darker markings on its wings and body. Photos by A.C. Magyorosky USDA, Bugwood.org.
How can I control BCTV?
BCTV is a notoriously difficult disease to control, but with integrated pest management, great strides can be made in reducing the harm caused by this disease. Because BCTV is spread exclusively through the feeding of an infected beet leafhopper (BLH), or Circulifer tenellus, controlling the disease hinges around the understanding of BLH biology.
The BLH most commonly overwinters and breeds on desert weeds in early spring and moves into agricultural areas across various habitats in late spring to early summer. 1 to 2 generations can be produced throughout the summer months, with the last brood typically migrating back to warmer climates for overwintering5. So, what are the best things you can do to control BLH and BCTV?
Remove Infected Plants
Once a plant is infected with BCTV, there is no known way to cure the plant and no varietal resistance to this disease has yet been discovered. To help reduce the rate of infection, remove the infected plant as soon as possible. In the early months of the growing season, having seedlings on hand to backfill can help maintain expected harvest yields.
Try a higher planting density and/or shade cloth or row covers
Beet leafhoppers prefer to feed in full sun, where vegetation is sparse and allows heat to penetrate through the plant canopy5. Higher density planting can help make your field less attractive to the beet leafhopper and also allow for more economic cushion if plants need to be removed due to BCTV infection7.
Shade cloth has been observed to reduce BCTV infection rates in other crops and is likely effective due to the beet leafhoppers affinity to full sun. Similarly, row covers can provide shade and, additionally, area closure to prevent the leafhopper from accessing your plants all together. Even if you have to remove the cover as the plants get bigger, protecting the plants in early to mid-season relieves exposure when feeding pressure is highest7 and potentially reduces plant susceptibility to infection since the plants will be more mature once the covers are removed9.
High density planting may reduce the susceptibility of your hemp crop to BCTV. Photo from Netafim USA.
Apply government-sanctioned insecticides
Insecticide application can help control BCTV occurrence in some crops and not in others. Due to the known unfavorable taste of tomato to the beet leafhopper, there is no evidence that insecticide application reduces the occurrence of BCTV in tomatoes7. In sugar beet, on the other hand, occasional systemic insecticide application has been proven to reduce BCTV8.
There has been no research yet conducted on the use of insecticide application on BCTV reduction in hemp or whether the beet leafhopper has a preferential taste for the crop, so the verdict is still out on the effectiveness of insecticide application.
While there are numerous organic and inorganic options that can successfully reduce beet leafhopper populations and feeding, it is important to follow federal and state regulations to understand which of these insecticides are approved for use on hemp crops in your area.
Weed control in and around your field
BCTV can easily re-infect a field year after year by over-wintering on or near your operation. The beet leafhopper (BLH), can infect hundreds of different crops and weeds including both non-native and native species, many of which can be asymptomatic when infected. This generalist aspect of BLH biology makes over-wintering sites abundant, but there are a few actions you can take to reduce the availability of these pest and disease reservoirs.
Be familiar with the beet leafhopper’s favorite plants, which include kochia, lambsquarter, halogeton, Russian thistle, greasewood, four wing saltbrush6, filaree, hoary cress, tumble mustard, flixweed, and pigweed5. Managing weeds in and around your field, especially those that are highly favored by the beet leafhopper, can help reduce the occurrence of BCTV in your grow8. In warm climates, winter weed management is also necessary, as the beet leafhopper can take up year-round residence if summer and winter vegetation is adequate5.
Encourage predatory insects
Beet leafhopper eggs can be parasitized by up to 80 or 90% by several species of parasitic wasps including the Anagrus nigriventirs, Aphelinoidea zarehi, A. turanica, and Paracentrobia subflava. Additionally, common generalist insect predators such as the green lacewing, spiders, assassin bugs, and big-eyed bugs are likely to feed on beet leafhopper9 and many are commercially available for sale.
Early Detection, Rapid Response
The sooner you know that BCTV is in or near you field, the better prepared you will be to rapidly respond. Using EDRR’s Velox Drone Kit, you will be immediately enabled to have a daily eye on all the plants in your field. Currently, our technology produces high resolution maps of your operation with just a touch of a button. Given the drastic symptoms of BCTV, the disease can be reasonably detected through frequent imaging of your field. Find out more about the power of drone technology in agriculture here.
What is more, EDRR is actively designing artificially intelligent software to automatically detect the presence of BCTV in your field. Coming 2021, you will be able to autonomously fly your drone and get an annotated report alerting you to BCTV occurrences in your operation! Sign up for our newsletter to keep up to date on this amazing development!
Do you suspect that you have BCTV in your field? Contact us today and we can also help you get your plants tested to confirm the disease. Since hemp is such a new commodity and so little is known about BCTV in hemp, EDRR is also working to produce community resources to track the spread of the disease in the crop and to keep you up to date on the latest tools and management advice specific to Cannabis.
References
1 European Food Safety Authority Panel on Plant Health, Jeger, M., Bragard, C., Caffier, D., Dehnen-Schmutz, K., Gilioli, G., Gregoire, J., Miret, J.A., MacLeod, A., Navarro, M.N., Niere, B., Parnell, S., Potting, R., Rafoss, T., Rossi, V., Urek, G., Van Bruggen, A., Van der Werf, W., West, J., Chatzivassiliou, E., Winter, S., Hollo, G., & Candresse, T. (2017). Pest categorization of Beet curly top virus (non-EU isolates). EFSA Journal, 15 (10). doi.org/10.2903/j.efsa.2017.4998
2 Giladi, Y., Hadad, L., Luria, N., Cranshaw, W., Lachman, O., & Dombrovsky, A. (2020). First Report of Beet Curly Top Virus Infecting Cannabis sativa in Western Colorado. Plant Disease, 104(3). doi.org/10.1094/PDIS-08-19-1656-PDN
3 Davis, R.M & Wang, H. (1998). Curly top virus found in perennial shrubs in foothills. California Agriculture, 52(5). doi.org/10.3733/ca.v052n05p38
4 Nachappa, P., & Chiginsky, J. (2019). Mites that Feed on Hemp- Fluid Feeders: Beet Leafhopper and Beet Curly Top Virus. Colorado State University. Retrieved from https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Curly%20Top%20Beet%20Leafhopper%202020.pdf
5 Munyaneza, J.E., & Henne, D.C. (2013) Insect Pests of Potato: Leafhopper and Psyllid Pests of Potato. doi.org/10.1016/C2010-0-67998-3
6 Schalau, J. (2013) Management of Curly Top Virus. Agriculture and Natural Resources University of Arizona Cooperative Extension. Retrieved from https://cals.arizona.edu/yavapai/anr/hort/byg/archive/curlytopvirus2013.html
7 Heflebower, R., & Schalau, J. (2014) Controlling Curly Top Virus in Tomato. Horticulture Extension of Utah State University. Retrieved from https://digitalcommons.usu.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1634&context=extension_curall
8 University of California Agriculture and Natural Resources. (2016) UC Pest Management Guidelines: Sugarbeet: Beet Leafhopper. Retrieved from http://ipm.ucanr.edu/PMG/r735300611.html
9 Wang, H., Gurusinghe, P.A., & Falk, B.W. (2007) Systemic Insecticides and Plant Age Affect Beet Curly Top Virus Transmission to Selected Host Plants. Plant Disease, 83(4). doi.org/10.1094/PDIS.1999.83.4.351
10 Rondon, S.I., & Oppedisano, T. (2020) Biology and Management of the Beet Leafhopper and Purple top Phytoplasma Affecting Potatoes in the Pacific Northwest. Oregon State University Extension Office. Retrieved from https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9282.pdf
Do you suspect BCTV on your farm? Where do you live? Are you interested in getting your plants tested? Let’s work together to get an interactive map going of active BCTV!