If you have a tomato plant wilting in your garden, it may not be from the heat or a lack of water. There's a relatively common virus disease of tomatoes, peppers and beans that mimics symptoms of moisture stress. It's called Curly Top, and as the name implies, one of the symptoms is curling leaves.
In addition to curling, the leaves become thickened and stiff. Leaves twist and curl upward. They may remain green or turn yellow with purple veins. As with other virus diseases, plants become stunted. Fruit may be deformed or fail to develop. Individual tomato, pepper, and bean plants can be infected and show symptoms while plants next to them may show no symptoms and be virus-free. The disease spreads in a random fashion.
Determining Curly Top
The best way to determine the difference between wilting from lack of water and wilting from curly top virus is to simply water. Soak the soil around the distressed plant in the early evening. Check the plant the following morning. If the plant has not revived overnight, it's likely the plant has curly top.
There is no chemical controls for curly top. The virus is spread from diseased to healthy plants by an insect called the beet leafhopper. It's a very small, wedge-shaped, winged bug only one-tenth of an inch long. It's color varies from pale green to gray to brown.
Given their size, beet leafhoppers are very hard to detect, but there are things you can do to limit their contact with your plants and thus prevent the spread fo the disease.
Curly Top Prevention
Because beet leafhoppers prefer to feed in sunny spots, providing some shade for your tomatoes and peppers will discourage leafhopper feeding. Shade cloth can be placed over plants using stakes. Not only will the shade discourage leafhoppers, it will also help tomato and pepper plants grow better and produce more fruit during the hottest summer months. Shade cloth can be purchased at most garden centers and home supply stores.
If a plant shows the wilting and stunting symptoms of curly top, remove the plant. Leaving a disease plant in place will provide a source of virus for feeding leafhoppers to pick up and carry to healthy plants. Put disease plants in a garbage bag, seal it up and put it in the trash.
The garden soil for growing tomatoes and peppers should be kept evenly moist. That isn't always easy to achieve under extremely hot and dry conditions. It may require daily or twice daily watering when temperatures are above 90 degrees. It's a good idea to apply a thick surface mulch of organic matter to reduce evaporation and level out moisture. The best mulches for garden beds are straw or compost. Straw can be purchased at area feed stores by the bale or loose. If you need a small amount, take a grocery sack or trash bag along and ask to purchase a bag of the loose straw. Compost can be purchased in bags at most garden stores.
To keep tabs of soil moisture, invest in an inexpensive moisture meter. Use the meter to test the soil on a daily basis. After a few days of testing, you'll know how often you need to water to keep the soil evenly moist. When the Monsoon Season begins, adjust your watering taking into account the higher humidity and welcome rainfall.
June and July are the months that require the most attention to water needs. Hot, dry conditions sap plants of moisture, stressing them and creating problems for the formation of flowers and fruit. Check soil moisture levels periodically, and make watering adjustments as necessary.