Straight Answers to Curly Questions
July 24, 2021
Last week, I attended an online conference with a few dozen researchers who all had one thing in common: they all study the curly top virus. Some of them were virologists who work on all kinds of viral pathogens. Others were entomologists who study how the virus is transmitted from plant to plant via the beet leafhopper. Some study this virus specifically in sugar beet production. Others study how it affects spinach or chile. This summer at the NMSU Agricultural Science Center at Los Lunas, we’re studying tomatoes, another curly top victim, and how shade cloth might help home gardeners by either protecting plants from the tiny, jumping insect vector (tiny, as in a skinny grain of rice) or by reducing sun and heat stress on the plants and thereby reducing water requirements.
I’m lucky to have a dedicated team of volunteers who are helping with the tomato study fieldwork. At the curly top research meeting, I presented our work and asked the experts to answer questions from the volunteers. For this week’s column, I’ve selected a few that I think readers will find interesting and, hopefully, useful in their own gardens.
Before I get into the Q&As, we need more background on the virus, the plants affected, common symptoms, the insects that spread it, and where these insect vectors live. The following excerpts were taken from the newly revised NMSU Extension Guide H-106: Curly Top Virus by Dr. Natalie Goldberg and Phillip Lujan.
“Curly top virus (CTV), or beet curly top virus (BCTV) as it is more formally known, is widespread throughout arid and semiarid regions of the world. The virus is common in the western United States from Mexico to Canada and in the eastern Mediterranean Basin. The virus has a wide host range, causing disease in over 300 species in 44 plant families. The virus appears to be restricted to broadleaf plants because no monocotyledonous plants have been identified as hosts for this virus. The most commonly infected hosts include sugar beets (for which the disease was first named), tomatoes, peppers, beans, potatoes, spinach, cucurbits, cabbage, alfalfa, and many ornamentals. The virus also survives in many weeds, such as Russian thistle (tumbleweed) and mustards (London rocket and flixweed).
“Symptoms vary depending on the host; however, this disease produces some general symptoms such as curling and twisting of the leaves… Severity of disease is also dependent on the age of the plant when infected. For example, when young plants are infected they will often die shortly after infection. When plants are infected after the seedling stage, the plants survive but are yellow and stunted. Infected leaves of some hosts, particularly tomatoes and peppers, become thickened and crisp or stiff, and roll upward as the petioles curve downward. The leaves turn yellow with purplish veins. Leaves of other hosts such as beets become very twisted and curly. In most cases, yield is reduced, and the fruit that is produced ripens prematurely. The immature, dull, and wrinkled fruit is a good diagnostic symptom for tomatoes infected with CTV. If plants are infected after they have begun to set fruit, it is not uncommon to see infected and healthy fruit on the same stem.
“This disease is transmitted (vectored) from infected to healthy plants by a small insect called the beet leafhopper (Circulifer tenellus). The leafhopper is an effective vector because it is able to transmit the virus after feeding on an infected plant for as little as one minute and can subsequently transmit the virus for the remainder of its lifetime (persistent manner). The virus is not passed on to leafhopper progeny.”
In addition to several host crop plants, the beet leafhopper is known to feed on several common weeds, including the winter annual London rocket. Therefore, weed control is a recommended way to make your yard, garden, or fields less hospitable for new generations of this pest.
More from the aforementioned NMSU Extension publication: “There are no chemicals available for controlling the virus, but several cultural practices can help reduce or eliminate infections. Although resistance to curly top is not known, growers may benefit from planting several different cultivars since some may be somewhat tolerant of the virus. Good sanitation practices, such as weed and insect control, are also essential for limiting the occurrence of the disease. Home gardeners may also consider planting susceptible hosts, such as tomatoes and peppers, in a slightly shaded part of the garden since leafhoppers prefer to feed in sunny locations. If the garden is in full sun, it may be helpful to place a netted cage over the plants when they are young. This netted material will provide a small amount of shade and, if the holes are small enough, will provide a physical barrier to prevent leafhoppers from getting to the plants.”
Again, more details can be found by visiting Guide H-106: Curly Top Virus. And you can find several related Southwest Yard & Garden columns by searching for the term “curly” on my Desert Blooms blog or the SWYG Archives.
Can other insects also transmit the virus?
No. While other insects may ingest the virus when feeding on your plants (I’m looking at YOU, tomato hornworm), they do not harbor the virus or transmit it to other plants.
Can the virus be transmitted by human touch?
We can all breathe a sigh of relief on this one because the virus is not transmitted by touch or by pruning shears either.
Can infected plants be asymptomatic?
Yes! Whether or not symptoms fully develop is particularly dependent on both the plant's growth stage when it becomes infected and by how healthy the plant is. Healthy, unstressed plants that are infected later in the season may be big and strong enough by then to fight off the infection on their own and still produce a good crop.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!