Disease can devastate a tomato crop quickly. Here are some of the common ones and some of their controls.
Early blight, a fungal disease, is a common problem east of the Mississippi and in the Far West. The first signs of trouble appear on the lower leaves as small brown spots with concentric rings in their centers and yellow margins. Affected leaves eventually turn yellow and fall from the plant. Fruits may also be affected. Warm, humid weather favors the spread of this disease.
To help keep early blight in check, clean up all tomato plant remains at the end of the season -- disease-causing spores can survive over the winter on plant debris. Mulch plants to reduce splashing spore-carrying soil onto leaves during rains, avoid wetting the foliage when you water, and make sure there's good air circulation around plants by not crowding them.
Some chemical fungicide sprays can control early blight if applied regularly when weather conditions are favorable for the spread of the disease. Check with your local county Extension agent to find out what fungicides are recommended in your state. Be sure you read the label and follow all the directions carefully when using any pesticide.
Late blight is another fungal disease that affects tomatoes in the same geographical areas as early blight. Leaves develop bluish gray spots, then turn brown and drop. Fruits develop dark brown, corky spots. Wet weather with warm days and cool nights sets the stage for this disease, which also infects potatoes.
To control late blight, avoid wetting the foliage when you water, use an approved fungicide regularly and clean up tomato plant debris in the garden at the end of the season.
Leaf spot can be caused by several different kinds of fungi. It's often a problem in the southeastern states and some northern areas that have warm, moist weather. Septoria leaf spot is common in the Northeast, mid-Atlantic and Central states; gray leaf spot is common in the Southeast. Septoria begins with many small brown spots with black specks in their centers; older leaves are affected first. Eventually infected leaves turn yellow and drop. Gray leaf spot is similar, except that the spots have gray centers.
The fungi that cause these diseases live on old tomato plant debris in the soil and nearby perennial weeds. Rotating crops from one spot to another in the garden each year is one way to keep this disease in check. Clean up the garden well at the end of the season, avoid overhead watering and apply an approved fungicide regularly.
The Wilt Diseases
Verticillium and fusarium wilts are two troublesome soilborne diseases for which there is no cure. Fusarium causes yellowing of the leaves, wilting and early death of the plant. Verticillium causes similar symptoms, but seldom kills tomato plants. Growth slows down, plants lose leaves and fruits may develop sunscald because of poor foliage cover. The best prevention is to grow verticillium- and fusarium-resistant varieties.