Tomatoes subject to many diseases

Last week, I wrote about tomato spotted wilt virus, bacterial wilt and tomatoes that roll their leaves. I would like to continue discussing some of the most common problems with tomatoes.

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There are a number of diseases, cultural and environmental problems that can hurt tomatoes.

One disease problem is early blight. The name says a lot about this disease. It occurs early in the growing season for tomatoes. The disease likes a temperature range of 75 to 85 degrees and wet conditions. This year, we had many rainy days in the 70s. Early blight can occur on all parts of the plant except the roots and can cause injury during any stage of a plant's development.

The older foliage usually shows the first symptoms, as small brown to black lesions form on the leaves. Often, a yellow halo will surround the spot, and I have seen large areas of the leaf turn yellow from just a few spots. As the spots get larger, concentric rings might become apparent.

Late in the season, lesions multiply and the plants might lose their lower leaves. If the disease is bad enough, all of the leaves on the plant can be lost.

The best way to control this disease is by fertilizing properly, as high soil fertility reduces the severity of early blight.

The next way is to use mulch around your plants. When water hits bare soil, it can splash spores of the fungus onto the leaves of the plant. This is how early blight gets started.

You also need to ensure good air circulation around the plants. You can do this by increasing the distance between them. The air movement helps dry the plants.

One of the most common calls that I get about tomatoes is about a nutrition problem -- blossom end rot. Blossom end rot appears as a dry, leathery spot on the blossom end of tomatoes. It also can affect peppers and watermelons. The spot is usually slightly sunken. Other rots can infect this spot.

Blossom end rot is caused by a lack of calcium. By the time the tomato reaches the size of a nickel it has most of the calcium it will ever have. This is why we need to prevent blossom end rot early. Inadequate water supply, low pH or low soil calcium levels can cause this problem.

Last spring and summer, I had to throw away about half of my tomatoes because of blossom end rot. I had plenty of calcium in the soil, but it was unavailable to the plant. Calcium has to move into the plant with water. I didn't have a good way to water my plants, so I had a bad case of blossom end rot.

You can control and prevent blossom end rot by watering the plants well and letting the soil dry between waterings. A good watering schedule is three-quarters of an inch twice a week if there is no rain.

Mulch will help conserve moisture levels in the soil. Apply a 2- to 3-inch mulch layer around the plant.

The best way to determine the amount of calcium in the soil is to have your soil tested. The sample will tell you how much lime and fertilizer are needed. If you haven't had your soil sampled, you can add gypsum (calcium sulfate) or lime to the soil at planting. Mix a cup in each planting hole or use one pound per 100 square feet. Apply this once you see the problem, but these treatments work slowly. Plants often appear to grow out of the problem as growing conditions improve.

Avoid overfertilizing plants with high-nitrogen fertilizers. The soil sample will provide a recommended rate of fertilizer to use, and this is important, especially when it comes to ammoniacal nitrogen. Excessive nitrogen rates can reduce the uptake of calcium into the plant.

The last problem is an environmental one we can't do much about. This problem is tomato plants not setting fruit. Some fruit-setting problems can be associated with too much nitrogen fertilizer, but most are the result of high temperatures. Tomatoes set fruit when nighttime temperatures are below 85 degrees. When nighttime temperatures rise above this level, especially above 90 degrees, the plants will not set fruit.

If you keep your tomato plants healthy, you will see them set fruit as the temperature cools in late summer and early fall. Some varieties will set fruit during high temperatures. I have seen these in local garden centers with ''heat tolerant'' on their labels.

Even with all the problems that gardeners can have growing tomatoes, they are still the top-selling plant.

Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at charlesp@uga.edu. The extension Web address is www.ugaextension.com/columbia.