Tomato fungal infections can quickly turn a promising harvest into a row of wilted, dying plants. Tomato plant fungus can attack at any stage of the plant's growth and affect all parts of the plant. It can dramatically reduce your crop's yield by either robbing the plant of its ability to use photosynthesis for energy or by directly causing the fruit to rot on the vine. While fungicidal treatments may help, the best cure for tomato fungal infection is prevention.
Gray mold and ghost spot affect most plants, but plants become more susceptible to this fungus the older they get. It causes light tan or gray spots to form on the leaves; gray-brown fungal growth covers these spots. The leaves eventually wither, diminishing the plant's ability to perform photosynthesis to produce food. Tan, elliptical spots may also form on the stem at its junction with a leaf. These spots have concentric rings and may spread all the way around the stem. You can prevent this fungal infection by planting your tomatoes in well-drained soil. Plenty of room between plants and prevention of weed growth will provide good air circulation to help prevent gray mold.
Septoria blight is caused by the Septoria lycopersici fungus. This fungal infection generally shows up on the lower leaves first, after the first fruits form. It causes circular spots about 1/8 inch in diameter, with dark edges and tiny black spots in the centers. Affected leaves may turn yellow and fall off the plant, and a severe infection can cause leaves to drop so rapidly that the plant's fruit yield and quality also drop. You can help prevent Septoria blight by cleaning up all plant material after the last harvest in the fall, since the fungus requires this material to survive the winter. This fungus spreads in water, so work with your plants only when the leaves are dry.
Early blight is caused by the Alternaria solani fungus. It shows up on lower leaves first, forming large spots up to 1/2 inch in diameter. These spots are dark brown or black with concentric rings resembling a target. The leaves turn yellow around the spots and eventually fall off. When the temperature is between 75 and 85 degrees, early blight spreads quickly to the upper areas of the plant and can result in most of the leaves falling. Early blight can also attack the stem and fruit of the tomato plant. The fungus overwinters in tomato plant debris, so remove and destroy all plant material at the end of the season.
The fungus Phytophthora infestans causes late blight of tomato plants. It usually attacks during wet weather when the nighttime temperature is 50 to 55 degrees and the days do not warm up over 85 degrees. Late blight causes dark green to almost black spots that appear greasy or wet. These spots spread in from the edges of the leaves. The Phytophthora infestans fungus may also cause spots on the fruit. This fungus, as with the other blight-causing fungi, overwinters on plant material.
You can minimize your risk of fungal disease in your tomatoes by rotating them in your garden. Plant tomatoes in the same area of the garden only once every three or four years. Many fungi, as well as insect pests, survive the winter in leftover plant debris, so clean up and dispose of tomato plants after the last harvest in the fall. You can also cut down on the spread of tomato plant fungi by removing wilted or diseased leaves as soon as you find them and by working with your plants only when their leaves are dry. Watering your plants from the bottom instead of with an overhead sprinkler can prevent fungus from spreading in splashed water droplets. Fungal infections are difficult to fight once they've infected your tomatoes; for this reason, you should consider applying a preventive fungicidal spray as soon as your tomatoes produce marble-sized fruit.