When the leaves of your tomatoes, squashes, watermelons or other garden crops thicken and curl, plant growth is stunted and they eventually die, the cause might be curly top virus (CTV). The disease is also known as beet curly top virus (BCTV), named after the beet leafhopper, the insect that transmits the virus.
Curly top virus is an extremely destructive plant disease that affects more than 300 plant species, both edibles and ornamentals. How badly your plants are infected depends very much on their age. Seedlings and young plants are especially susceptible. The weather conditions play a big role in how fast curly top spreads.
Identifying the problem promptly and removing infected plants can help you minimize the spread of the disease.
Which Plants are Susceptible
The garden crops mostly affected by curly top are tomatoes, sweet and hot peppers, beans, potatoes, spinach, any members of the cucurbit family such as watermelon and squash, cabbage, but also ornamentals.
Weeds such as Russian thistle (tumbleweed) and mustard are preferred hosts of the beet leafhopper so controlling those is crucial in managing curly top.
What Causes Curly Top
When the beet leafhopper (Circulifer tenellus) feeds on a plant infected with curly top, it takes less than one minute for the insect to pick up the virus. The incubation period—the time until the insect transmits the virus to other plants–depends on the temperature. It ranges between four to six hours in hot weather and 21 hours in moderate temperatures.
How the Disease Develops and Spreads
Once a plant is infected, it depends again on the temperatures when it will develop the first symptoms. In hot weather, it takes 25 hours, in moderate temperatures 14 days, and in cool weather 30 days.
While high humidity makes many other plant diseases spread faster, high humidity is actually a good thing for curly top. The beet leafhopper is less active in relative humidity above 50 percent. In low humidity, on the other hand, the beet leafhopper is out and about. That’s why the disease is especially widespread in arid and semi-arid regions like the western United States.
How to Identify the Beet Leafhopper
The beet leafhopper is about one eighth of an inch long and wedge-shaped. Its color is pale green, gray or brown, with dark markings on its upper surface.
The insect overwinters in weeds such as Russian thistle (tumbleweed) and mustard. In the spring, the adults lay eggs on their hosts. After the eggs hatch, it takes two to three months for the nymphs to develop into adults.
When disturbed, the beet leafhopper jumps or flies away. If the insects don’t find suitable hosts on which to feed, they move to other plants. The damage caused by their feeding on plants is negligible.
How to Identify Curly Top
There are several symptoms that may indicate curly top:
- The leaves of the infected plants, especially tomatoes and peppers, twist and curl upwards. The petioles (the stalk that is attached to the stem and is supporting the leaf) may bend downwards.
- The leaves turn yellow with purplish veins. Tomato leaves become thick and leathery.
- Older plants may show stunted growth.
- The yield is reduced, and fruit ripens prematurely and is deformed, looking dull or wrinkled. It can also haven an odd taste. Eventually the plant dies.
Curling leaves does not necessarily mean that your garden crops have curly top, it could simply be lack of water so make sure that you water your garden regularly.
Tomato spotted wilt virus, another disease affecting tomatoes plants, might be confused with the symptoms of curly top. But you can tell the two apart by closely inspecting the leaves: the upper young leaves of a plant that has tomato spotted wilt are bronze-colored with small dark spots or flecks, whereas the leaves of plants infected with curly top do not have any.
Treating and Preventing Curly Top
There is no chemical to treat plant with curly top virus, and an insecticide won’t control the leafhoppers that transmit curly top virus. By the time you apply an insecticide in one location, they will have likely moved on to a different faraway location already. But there are other measures you can take to control the virus.
- Make sure that the seedlings or transplants you put into your garden are healthy and virus-free. At present, there are no virus-resistant tomato varieties.
- Remove diseased plants immediately, and throw them in the trash. The beet leafhopper can go through several generations in a year, and may have already laid its eggs on the plants.
- Mechanically remove all annual and perennial weeds, especially the preferred hosts Russian thistle and mustard weeds, in addition to cleaning out any spent vegetable plants in the fall. This is crucial to prevent beet leafhoppers from infesting your garden in the next growing season. Any weedy wasteland in the vicinity of your garden might also be a place for the insects to overwinter.
- Remove sugar beets, as they are another preferred host of the beet leafhopper.
- Use fine mesh or netting to protect crops like tomatoes and peppers, and to help deter the beet leafhoppers from feeding on the plants and infecting them with curly top virus.