Issue: July 18
Curly top virus strikes again!
Q. I have 7 tomato plants that are becoming yellow and the leaves are curling up. What is wrong? What can I do?
A. You are probably describing curly top virus. I have talked to many people who are seeing the same symptoms you have described. It is common in tomatoes, chiles, beans, and some other garden vegetables. I have personally lost about 10 tomato plants and one chile plant to this garden nemesis. Last year people lost many plants to curly top while none of my plants were affected. This year my garden is severely affected.
How can a disease like curly top be so unpredictable? The reason is that this viral disease (plant virus that does not infect animals) cannot live in the soil over the winter. It must be in a living plant to over-winter. It is then spread from the winter host plant (which may not show symptoms, or be injured) to the vegetable garden by a small insect called the beet leaf hopper. The beet leaf hopper feeds by sucking juices from the host plant and "hops" to the vegetable garden where it feeds on our tomatoes, chiles, and other plants. As it inserts its feeding tube into the new plant, it injects a small amount of "saliva". The virus may be in that small quantity of injected material. If the virus is injected into the garden plant, and if the garden plant is susceptible, then the plant will develop disease symptoms.
Curly top symptoms are stunting of the plant, yellowing of leaves, development of purple coloration in the veins, and twisting and down-turning of the petioles that attach the leaves to the stem. The plant may persist in an "ill" state for a while, but new tomato or chile fruit production will cease. Any fruit already on the plant are safe for you to eat, but it is often wise to remove the plant so that the disease cannot be spread to other plants.
There is currently no known chemical treatment to kill or stop curly top virus. Treatments with insecticides to control the spread of curly top virus are usually not effective. The insecticide may cause the insect to more rapidly spread the disease before the insect dies. The best management strategy has been to manage weeds and other host plants in the garden. Over-wintering weeds, or perennial plants that die back, and then regrow from underground portions may also support the virus through the winter. One of the best known over-wintering hosts are the mustard weeds. Managing the weeds so that they are not present when the garden is planted is one method of reducing the incidence of curly top virus. Dr. Ron Walser, NMSU Extension Small Farms Specialist, tried several treatments to protect tomatoes last year. He sprayed white kaolin clay on some plants to "hide" the plants from the insects carrying curly top viruses. This treatment was not very effective. He covered some tomato plants with floating row cover fabric (available in some garden supply stores). This treatment was extremely effective. It prevented the insects from having access to the tomato plants. The plants that he neither treated with kaolin clay or row cover fabric were almost completely affected and produced essentially no tomatoes. The "exclusion"Â technique was very effective and is recommended. Weed management during the winter is also a highly recommended technique, but there may be desirable plants in the landscape that show no symptoms and serve as hosts, so exclusion with row cover fabric remains a good idea for home gardens.
More information about curly top virus is available at the NMSU College of Agriculture, Consumer, and Environmental Sciences web site at http://aces.nmsu.edu/pubs/_h/h-106.pdf. Your local NMSU Cooperative Extension Service agent can help you confirm curly top virus if you think your plants are infected.
For more gardening information, visit the NMSU Extension publications World Wide Web site at http://aces.nmsu.edu/pubs/_h.
Send your gardening questions to:
Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd. SW
Los Lunas, NM 87031
Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.