Every year we face a number of pests that try to spoil the production of vegetables and fruits that we desert gardeners grow. Viruses attacking our tomatoes are among those pests. Two primary agents are Beet Curly Top Virus (BCTV)—also known as Curly Top—and Tomato Yellow Leaf Curl Virus (TYLCV).
Beet curly top virus
BCTV is transmitted from plant to plant by the Beet leafhopper (Circulifer tenellus). An infected adult leafhoppers might land, probe feed, and lay eggs on many different plants, including tomatoes, beans, pepper, spinach, squash and various weeds, but generally prefers beets.
Plants begin to show symptoms about 7 to 14 days after they are first infected. Leaves of infested plants are dwarfed, crinkled, rolled inward and cupped upward. Roots are stunted and may exhibit a proliferation of secondary rootlets.
Phloem tissues become necrotic and appear as dark rings when viewed in cross section. Plants do not grow, their stems become stiff. Generally, the fruit on the plant ripens prematurely and are deformed. Veins on the underside of leaves will usually have a purple discoloration, and may be roughened. These often produce swellings or spine-like outgrowths.
Tomato yellow leaf curl virus
The tomato yellow leaf curl virus (TYLCV), also referred to as tomato leaf curl, is a viral disease that is usually transmitted by sucking insects and causes similar leaf roll symptoms as BCTV. This disease is a geminivirus and although it can be found on a variety of plants—chillies, eggplants, beans, nightshade and lisianthus, to name a few—the tomato is its favorite. In particular, this disease tends to be more of a problem in greenhouse-grown tomatoes than those grown outdoors.
Sometimes plants can develop infections yet not show obvious symptoms. Flowers may drop from the plant, but their shape remains unaffected. Depending on the time of infection, flowers may fail to set fruit even though they remain on the plant (early infection generally inhibits fruit production).
If the infection occurs while fruit is developing, the fruit will ripen in a nearly normal manner. If the infection occurs while the plant is still young, the plant will remain small, with terminal and auxiliary shoots remaining upright. Leaves that develop soon after the infection occurs will cup downward, but later leaves will appear pale green or yellow and deformed, with leaf edges cupping upward.
Controlling the virsuses
Unfortunately there is no cure for BCTV or TYLCV. Instead, control is prevention for this virus. First of all, it is vital to examine the plants and how they have been grown to determine whether it is an actual disease or it is environmental. For example, the cause is likely to be environmental stress or herbicide injury if an entire section of plants exhibit leaf-roll symptoms. Insects just aren’t that thorough, some of the plants will escape the infection.
Trying to control curly top (caused by BCTV) is difficult and efforts to breed resistance to curly top into tomatoes have not been successful. Spraying tomatoes with insecticides does not control the disease because leafhoppers migrate from distant places and do not remain in one area long.
By the time symptoms of curly top become evident, the leafhoppers have moved away to other crops. As such, transmission of BCTV from an infected plant to a healthy one is uncommon and removing symptomatic plants to prevent further disease transfer does nothing in the battle against these viruses.
There is no positive control for viral leaf curl (caused by TYLCV). Sucking insects, such as sweet potato whiteflies and aphids, should be controlled since they are insect carriers of the disease.
Application of products that repel insects will reduce the sucking-insect count on your crop and, therefore, increase the odds that your plants can avoid the disease. Keeping areas weed-free will also reduce the odds of the virus being spread because plants sources for the disease are often wild.
Removal of symptomatic leaves, stems and fruits will not eliminate TYLCV from otherwise healthy looking plants. Conversely, this might actually lead to further spread of the disease through infected tools, so thoroughly clean tools and other equipment that may have come in contact with the diseased plants with a one to 20 solution of bleach and water.
In general, keeping plants as healthy as possible will build their immunity and make them less susceptible to insect attack. Avoid unnecessary injury during pruning and cultivating. Keep the weeds to a minimum, and try using insect repellent mulch like “cedar”, and keep some garlic barrier around. Keep plants as well spaced as is practical and most of all; good luck!