Even though tomato plants are bred to resist certain common maladies, none are immune to problems. Weather plays a part in the spread of viral and bacterial infections, and garden sanitation can have an effect on soil-borne diseases. Some pathogens are brought home with purchased plants and are transplanted to the garden. Other disease organisms can arrive over time in the soil or compost pile. Being alert for the first signs of a problem is the best way for gardeners to keep tomato plants healthy and productive.
Verticullum and fusarium wilts are caused by a number of different soil-borne fungi. Without microscopic evaluation, it is difficult to tell the two wilts apart. The fungus spores remain in the soil for many years. Spores can be transmitted by picking them up on your shoes or on garden tools. They attack tomatoes, strawberries and other garden plants by preying on plants with stressed root systems. Once the plant's roots are infected, the damage is transported through the vascular system, affecting lower leaves and branches first. Symptoms are yellowed, curled or red-veined leaves. The plant may begin to defoliate from the lower branches up as the disease progresses. Good garden sanitation is imperative to slow the spread of verticullum and fusarium wilt. Infected tomato plants should never be chopped into the soil or used in the compost pile. Remove and destroy them. Avoid planting tomatoes in soil where former plants were infected, including non-tomato plants. Crop rotation with non-susceptible plants may help limit the soil fungus. There is no cure for verticullum or fusarium wilt, but some tomato plant varieties are resistant. Look for plants or seeds with the notation "VF" which represents resistance to verticullum and fusarium fungi.
Leaf spot, or alternaria, is a fungus blight that occurs on tomatoes, usually in hot weather. It is caused by a soil-borne fungus. It first appears as yellow spots with dark centers on the leaves. The leaves may drop and the fungus may spread to stems and fruits. Remove infected plants from the garden. Allow more air circulation between plants and avoid overhead irrigation. Sulfur dust applications may help control the spread of leaf spot.
Blossom End Rot
Blossom end rot appears as a dark or black spot on the blossom end of the tomato, even while it is still on the vine. The spot is actually decay setting in. The decaying spot opens the way for other pathogens and molds to enter the fruit, so you should remove affected tomatoes to keep the garden clean. Blossom end rot is caused by a lack of usable calcium in the soil. Too much nitrogen may be a culprit. The soil pH for tomatoes should be about 6.5. Adjust it and increase the calcium in the soil by lightly applying lime or bone meal.