Most problems that show noticeable signs and symptoms on tomato plant leaves are caused by fungi. To prevent the same problems year after year, it's best to rotate plants to various locations throughout the garden.
Fellow gardeners or your county extension service will alert you to the common diseases to watch out for in your area. Try to buy disease-resistant varieties. According to "Rodale's All-New Encyclopedia of Organic Gardening," these are often noted in the name of the cultivar.
Fusarium oxysporum is a fungus that spends the winter months in garden soil, and commonly affects a wide variety of vegetable and flower beds year after year. It favors warm soil and air temperatures. Leaves on the lower part of the plant appear dry and wilting, though you're giving the plant plenty of water.
After the plant dies, cut it off at an angle near the base and look inside the stem. If you see streaks of brown, Fusarium wilt killed the plant. No chemical solution exists. To prevent it next year, plant resistant varieties in a different location, and throw the dead plants and leaves in the trash--not the compost bin.
Verticillium dahliae and Verticillium albo-atrum (collectively called verticillium wilt) are also a soil-borne fungi, but they prefer cool, moist weather. Signs of the disease are most obvious when the weather dries and warms up. The symptoms are often confused with those caused by some bacterial wilt diseases. Older leaves at the bottom of the plant look yellow, wilting and dry, but those on the ends of stems remain unaffected.
Cut through the infected stem. If verticillium wilt is to blame, you'll see a discolored ring inside. The plant won't die, but will produce small fruit that doesn't taste very good. You can't control verticillium wilt with chemicals, so plant resistant varieties if available.
The fungus Alternaria solani causes early blight in tomato plants that are poorly fertilized and frequently watered from above late in the season. Affected plant leaves show small spots, yellow on the outside and black in the middle. Eventually, leaves dry up and no longer protect fruit from the sun. Spray fungicides or copper soaps early in the season to aid control. Don't compost affected plants or rotate crops and try resistant varieties.
Septoria Leaf Spot
Spots on lower leaves, smaller than those caused by early blight, may be signs of the fungus Septoria lycopersici. The disease affects only tomato plants, but overwinters on infected weeds and seed. Warm, wet weather develops the disease, which can cause leaves to yellow, and defoliate the plant. Fungicides control Septoria if sprayed weekly during wet periods. Don't compost dead plants, but destroy them, and rotate crops annually.
Tomato Leaf Roll
Leaves of tomato plants during times of heavy rain may curl up lengthwise and appear stiff. Don't worry, these symptoms are characteristic of tomatoes growing in wet soil. The plant may look unhealthy, but the fruit will be unharmed. Once the soil drains, the plant should appear normal again.