Tomato Yellow Leaf Curl

Tomato yellow leaf curl is a disease of tomato caused by Tomato yellow leaf curl virus. In March 2007, it was identified for the first time in California and currently has a limited distribution. An educational brochure (PDF) was created at that time and is available to print.

Infected tomato plants initially show stunted and erect or upright plant growth; plants infected at an early stage of growth will show severe stunting. However, the most diagnostic symptoms are those in leaves.

Leaves of infected plants are small and curl upward; and show strong crumpling and interveinal and marginal yellowing. The internodes of infected plants become shortened and, together with the stunted growth, plants often take on a bushy appearance, which is sometimes referred to as 'bonsai' or broccoli'-like growth. Flowers formed on infected plants commonly do not develop and fall off (abscise). Fruit production is dramatically reduced, particularly when plants are infected at an early age, and it is not uncommon for losses of 100% to be experienced in fields with heavily infected plants.

Tomato yellow leaf curl virus is undoubtedly one of the most damaging pathogens of tomato, and it limits production of tomato in many tropical and subtropical areas of the world. It is also a problem in many countries that have a Mediterranean climate such as California. Thus, the spread of the virus throughout California must be considered as a serious potential threat to the tomato industry.

There are a number of factors why it has not yet spread to all the major tomato-producing areas of California, including the Sacramento and San Joaquin valleys. First, its vector, Bemisia whitefly species are not typically found in these tomato-producing areas because it is intolerant of winter temperatures there. Second, the Central Valley's winter season provides a 'natural' tomato-free period, which usually goes from late November through early February. Although the virus can infect other plants, tomato is the host in which it builds-up most quickly. Thus, by having an annual 'tomato-free period', it is likely that the amount of viral inoculum (as well as whitefly populations) will be significantly reduced by the time the tomato planting season starts again in late winter-early spring. This would mean that, even if the virus is able to overwinter, it may take a long time to reach levels that cause economic damage.

Tomato yellow leaf curl virus is a geminivirus (family Geminiviridae). Although it can infect a relatively wide range of plant species, tomato is the host to which the virus is best adapted and that facilitates the build-up of the virus to high incidences in the field. Other hosts include solanaceous crops, which may develop symptomless infections, and weeds (e.g., nightshade and jimsonweed).

In addition, the virus causes leaf curl in certain varieties of common bean (Phaseolus vulgaris) and the ornamental plant lisianthus (Eustoma grandiflorum). A range of weeds from other families can be infected by this virus, but most of these do not develop obvious disease symptoms. It is not known how well whiteflies acquire the virus from symptomless hosts. However, it has been hypothesized that these hosts serve as a 'bridge' for the virus in the absence of tomato crops, and that perennial weeds help allow the virus to become permanently established.

The primary way the virus is spread short distances is by Bemisia whitefly species. Over long distance, the virus is primarily spread through the movement of infected plants, especially tomato transplants. Because it can take up to 3 weeks for disease symptoms to develop, infected symptomless plants could be unknowingly transported. The virus also can be moved long distance by virus-carrying whiteflies that are transported on tomatoes or other plants (e.g., ornamentals) or via high winds, hurricanes, or tropical storms.

Rapid and precise tests for Tomato yellow leaf curl virus are available at UC Davis and CDFA. These tests can be carried out in less than 24 hours. Anyone finding tomatoes with TYLC-like symptoms can contact their county farm advisor, Robert L. Gilbertson at UC Davis, or Tongyan Tian at CDFA.

Strategies to effectively manage the disease include:

Before Planting

• Select TYLCV-resistant varieties.
• Use virus- and whitefly-free transplants.
• DO NOT import tomato (or any potential whitefly host) transplants from areas known to have the virus (Florida, Georgia and Texas in the U.S.; and Mexico).

During the Growing Season

• Plant immediately after any tomato-free period or true winter season.
• Avoid planting new fields near older fields (especially those with TYLCV-infected plants).
• Manage WHITEFLIES.
• Cover plants with floating row covers of fine mesh (Agryl or Agribon) to protect from whitefly infestations.
• Rogue diseased plants when incidence of virus infection is low.
• Practice good weed management in and around fields to the extent feasible.

After the Growing Season

• Remove and destroy old crop residue and volunteers on a regional basis.
• A voluntary or enforced regional host-free period in areas lacking a true winter season (i.e., temperatures low enough to prevent crop cultivation and whitefly survival) might be a useful management tool. The crops to be included in a region will depend on the agroecosystem.

• Tomato Yellow Leaf Curl Virus brochure (PDF)