Hydroponic systems typically provide growers a major advantage over conventional outdoor gardening. Because this type of controlled gardening is usually isolated from outdoor pests, damage from pests is minimized. However, sometimes hydroponic systems are built where exposure to pests is indeed an issue. Every year growers battle pests that try to spoil the production of vegetables and fruits. Viruses attacking tomatoes are among those pests. Because gardeners are the type to keep trying, and do what it takes to succeed, we generally overcome these obstacles.

The rolling or curling of tomato leaves can be a symptom of environmental stress, herbicide damage or viral infection. If you suspect you're experiencing these problems, you will want to look into environmental stresses, such as overwatering or under-watering, too much fertilizer, or even the use of herbicides in the general area of your tomatoes. In this article I discuss the viral infections causing this symptom. Two primary agents causing this are beet curly top virus (BCTV) and tomato yellow leaf curl virus (TYLCV), both of which occur due to visiting insects.

Beet Curly Top Virus

The disease known as curly top, or beet curly top virus, might be found on tomato, bean, pepper, spinach, beet and squash plants. 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. The stems become stiff and plants remain stunted.

Generally, the fruits on the plant ripen prematurely and are deformed. Plants begin to show symptoms about seven to 14 days after they are first infected by a leafhopper. Visible veins on the underside of leaves will usually have a purple discoloration and may be roughened. These often produce swellings or spine-like outgrowths. Because tomatoes are not the only plants affected, you can see these symptoms in the squash plants pictured on page 50, and the insects that spread this disease actually prefer beets.

BCTV is transmitted from plant to plant by the beet leafhopper (Circulifer tenellus). Both the virus and the leafhopper have a wide variety of host plants. Once acquired by the leafhopper, BCTV is carried for the rest of the leafhopper's life. This is why such a long-distance spread of the disease is common. Adult leafhoppers may land and probe-feed on many different plants, but generally prefer to lay eggs on beets, tomatoes and various weeds.

Unfortunately, trying to control curly top is difficult. Efforts to breed resistance to curly top into tomatoes have not been successful and all tomato varieties currently available are susceptible. Spraying tomatoes with insecticides does not control the disease because leafhoppers migrate from distant places and do not reproduce or remain in tomato fields. As the symptoms of curly top become evident in tomatoes, the leafhoppers have long since moved on to other crops or weeds.

Tomato Yellow Leaf Curl Virus

TYLCV, also referred to as tomato leaf curl, is a viral disease that is usually transmitted by sucking insects. This disease tends to be a problem in greenhouse-grown tomatoes and is less common in plants grown outdoors. This disease is a gemnivirus, and although it can affect a wide variety of plants, the tomato is its favorite.

It might be found on crops of the Solanum family (tomato, pepper, chili, potato and eggplant), which may sometimes develop infections while not showing symptoms. It can also be found on weeds such as nightshade and jimsonweed. In addition, this virus causes leaf curl in certain varieties of common bean and the ornamental plant lisianthus. A range of weeds from other families can be infected, but most of these do not develop obvious disease symptoms.

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 chlorotic (yellow) and deformed, with leaf edges cupping upward. 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. If the infection occurs while fruit is developing, the fruit will ripen in a nearly normal manner.

TYLCV is transmitted by sap-sucking insects and causes similar leaf roll symptoms as BCTV in infected tomato plants. Purplish veins on the leaf underside will help to distinguish both of these viruses from physiological leaf roll and herbicide injury. Differing a bit from BCTV, with TYLCV, new leaves appear chlorotic (pale green or yellow) and cupped. In both cases overall plant growth becomes stunted and early infection usually inhibits fruit production. Unfortunately, there is no cure for either of these viruses.

Preventing Leaf Curl

First of all, it is vital to examine the plants and how they may have been treated to determine whether symptoms show an actual disease or environmental stress. If an entire section of a plant exhibits leaf roll symptoms, the cause is likely to be environmental stress or herbicide injury. Insects just aren't that thorough; some of the plants will escape the infection. For most other viruses, like the tomato mosaic virus, removing and destroying symptomatic plants prevents further transfer to healthy plants. However, transmission of either BCTV or TYLCV from an infected plant to a healthy one is uncommon, as the insects that brought the disease are usually gone.

There is no positive control for viral leaf curl. Sucking insects such as the sweet potato whitefly and aphids should be controlled since they are insect carriers of the disease. Application of products like a garlic barrier, which tend to repel insects in general, will reduce the insect count on your crop and therefore increase the odds of your plants avoiding the disease.

Plants that may have been the source for the disease are often wild, so keeping areas weed-free will reduce the odds of the virus being spread. Removal of symptomatic leaves, stems and fruits will not eliminate the virus from otherwise healthy-looking plants. This may actually lead to further spread of the disease through infected tools.

Thoroughly clean tools (using bleach and water at a ratio of 1:20) and other equipment that may have come in contact with the diseased plants. Sap on tools and plant parts can be an easy way to transmit the virus to other plants. Controlling large populations of aphids or other sap-sucking insects may help to reduce virus transmission to healthy tomato plants. Avoid unnecessary injury during pruning and cultivating.

Keeping tomato plants as healthy as possible will build their immunity and make them less susceptible to insect attack. Keep the weeds to a minimum, try using insect repellent mulch like cedar, and keep some garlic barriers around. Keep plants as well-spaced as is practical, and most of all-good luck!