Tomato plants are a relatively easy plant to grow, but they do have a couple of "weak spots" for disease that can leave your plants foundering and your fruits ugly and inedible. Fortunately, diseases in tomato plants are largely preventable just by controlling their environments in your home or yard. By taking proper care of your tomato plants and recognizing the signs and symptoms of problems early, you can keep your tomatoes happy, healthy and yielding for a long time.
Verticillium wilt, as the name implies, causes the leaves of tomato plants to wilt. At first, you may not notice because the older leaves will be the ones wilting and turning brown around the edges. Even watering will not halt this wilt. Left unchecked, the wilt will spread up the leaves of the tomato to the stem and will kill the plant. Generally this occurs in about midsummer. You can control verticillium wilt by removing affected leaves using sterile pruning and disposing of the debris in a garbage bag or by burning rather than allowing it to fall on the ground. Fungicides are also an option, but these must be used early in the season as preventatives rather than to treat a plant that already is bearing fruit, if you plant to eat the fruit. If the problem persists, you will need to remove the affected plant and either replace the soil in the area or avoid planting tomatoes in that area for four to six years.
There are a number of Verticillium-resistant cultivars on the market today, so you may want to consider investing in one of these for your garden in order to make the odds of your tomato plant catching this disease even smaller.
Anthracnose affects tomato fruits that are allowed to touch the ground or that hang near enough to the ground that the fungal infection can splash onto the fruits during a heavy rain. Anthracnose starts out as small, black spots, but it works quickly on tomatoes to destroy the entire fruit. Once on the plant, it can also go to work on leaves and stems, although this may take a little more time. You can control the infection through sterile pruning and, if worst comes to worst, by treating the entire plant with a fungicide.
Sun scald is an infection that occurs as the result of your tomato plant catching another disease. When the plant becomes defoliated from a disease like verticillium, the stems may turn brittle and brown, as if they have been scorched. This leaves the plant open to infection from any number of small, organic organisms that break down plant matter and can result in the death of your tomato plant in short order. If you are treating your plant for another disease, cover it with pine straw or place the plant in the shade to protect it from harsh, direct sun and prevent these infections. There is little that you can do to treat sun scald once it has occurred other than to try and prevent further damage.