Tomatoes are the most commonly grown vegetable in our area. Consequently, theirs are the most common vegetable problems in our area. Each year, I probably receive as many questions about tomato problems as all the other vegetable problems combined.
Tomato spotted-wilt (TSW) virus is a major disease. We have had it for many years, but the severity varies from one year to the next. Virus infected plants will develop curly, twisted leaves on the top third of the plant. The plant seldom dies, but it doesn’t produce many tomatoes, either. If your tomatoes experience the disease, your only option is to replace the affected plant with a TSW resistant plant. The TSW resistant varieties are also fairly heat-resistant. Most of the resistant varieties are determinate in plant type and will not reach a very tall height. Another option is to plant a fall tomato crop. Insects such as aphids and thrips vector or spread the disease from one affected tomato to another or from a weed plant to a tomato plant. The weed plants that host the virus and serve as a source of the disease are cool season plants. They are only around in the spring tomato season. Controlling the insects that spread the disease sounds like an attractive alternative, but 100 percent insect control is almost impossible.
Why are my tomatoes blooming but not setting fruit? The most common cause is hot weather. As we get into the summer and daytime temperatures exceed 90 degrees F, many tomato varieties will not set fruit. Blooming will continue, but fertilization of the bloom will not occur. Without fertilization, there is no fruit production. Certain “heat resistant” varieties will continue to set fruit in the early summer, but most varieties are not heat tolerant. In theory, if you can keep the tomato plant alive and healthy through the summer, it will bloom and set fruit again in the fall. This is not easy to do. Foliar diseases and insects continue to plague the plant whether it produces fruit or not. Most gardeners will not have the patience to continue management of a non-productive plant. It is more common to pull the plant out and plant again in late summer for fall production.
Round Up herbicide injury is very common. Tomatoes are very sensitive to Round Up and other herbicides. If even a minute amount comes in contact with a tomato plant it will cause adverse effects on production or kill the plant outright. Never apply Round Up as a herbicide in the tomato garden or around its borders. It is best to not apply Round Up in the vicinity of the tomato garden if there is any wind at all.
The most common group of foliar (leaf) diseases is caused by fungi. Anthracnose, early blight, late blight and grey mold cause spots on the leaf. Eventually, the entire leaf dies. This leaf loss usually progress from the lower to the upper portions of the stem. It is called “firing up”. Weekly applications of a recommended fungicide will prevent these diseases. These applications should begin when the first clusters of fruit form.
Soon I expect to begin getting calls on another common problem: blossom-end rot. That is to be expected when we consider the weather that we have experienced this spring. An extremely dry spring always precedes a large incidence of blossom-end rot. Blossom-end rot affects the fruit of tomatoes and bell peppers. On the bottom or blossom end of the fruit a black area will develop. The area can grow until it affects the entire bottom half of the fruit. When touched, the spot is tough and leathery. It is easy to distinguish blossom-end rot from other fruit rots caused by bacteria or fungi. Blossom-end rot is always tough to the touch. Other fruit rots are soft and often watery. The condition is caused when the plant is not taking up enough calcium. While this may be due to a deficiency of calcium in the soil, it can also and more commonly be caused by fluctuating soil moisture. It is always worse in times of drought. Many of the nutrient deficiency disorders that we see in our plants are caused by the plant’s inability to take the nutrient from the soil. Plant stress from pests and environmental conditions are often responsible for this inability. It is not only important to insure adequate fertility in the soil. A gardener must also provide for the other factors that affect plant health (soil moisture, weed control, insect and disease control, etc.).
The most effective preventative and treatment of blossom-end rot is to apply a solution of calcium nitrate or calcium chloride to the plant. This material is available at most garden stores under various trade names. When sprayed onto the leaves (foliar application), the calcium is absorbed into the plant and the deficiency is corrected. It is usually very effective. Begin applications when the first symptoms occur and continue at seven day intervals for a period of three weeks. These materials really help if applied in a timely manner. Supplementing calcium to the soil will help, but will be much slower to show a response in the plant. Unless there is a consistent and plentiful supply of soil moisture, the plant will not be efficient in uptake of the soil calcium. In other words, as long as the drought persists, it is more efficient to supply calcium by the foliar applications.