Tomato plants are susceptible to a variety diseases which cause foliage to turn yellow and brown. Some diseases may be carried from the greenhouse on purchased plants, or they may winter-over in the soil and compost pile in your garden.
Always remove infected tomato plants and destroy them away from the garden. Keep infected plants out of the compost pile. Good garden sanitation and crop rotation are the best methods for preventing yellow and brown leaf diseases on tomato plants.
Verticillium and Fusarium Wilt
Verticillium and fusarium wilt symptoms begin as yellow leaves at the bottom of the tomato plant. The yellow leaves progress to brown, and as lower branches die, the disease moves up the plant, affecting branches and leaves in its path.
Symptoms of the two wilts can be indistinguishable from one another unless a pathological study is done. Neither can be cured, and the solution for both is to remove affected plants from the garden. Do not compost the plants, as the wilts are soil borne. Crop rotation is the best defense against wilts. Allow a three year rotation with no tomatoes, peppers, or eggplants grown in the affected soil. Choose plant varieties labeled with VFN, VFNT or similar resistance that includes the V and F. The V denotes verticillium resistance; the F denotes fusarium resistance.
Early Blight (Alternaria)
Early blight is caused by a fungus, Alternaria solani. It begins as dark spots on the mature leaves. Sometimes the spots are target-like, with circles of yellow around them. The leaves turn yellow, then brown. Remove and dispose of affected foliage as soon as you notice early blight. If the tomato plant becomes defoliated, it is likely that the fruits will be overexposed to the sun and will suffer from sunscald. Sulfur dust is used to treat early blight with some success.
Avoid overhead watering, which spreads the fungus. More space between tomato plants allows better air circulation, which also helps prevent the spread of the fungus. The spores overwinter in plant debris, so use good garden sanitation. For a badly infected garden, plant tomatoes in a different place the following year.
Aphids and Whitefiles
Both aphids and whiteflies can cause minor leaf yellowing on tomato plants. You will know that these tiny insects are the culprits, because they leave "honeydew" on the leaves, which makes the leaves look shiny. Spray the tomato plants as needed with insecticidal soap to control both of these pests.
Cucumber Mosaic Virus
The cucumber mosaic virus is carried in tomato seeds in most cases. There is no cure. The virus causes the tomato plant to become stunted. The leaves turn yellow and brown as the plant dies. Destroy affected plants in an area away from the garden.
Herbicide damage from glyphosate (Roundup) can mimic the cucumber mosaic virus. Drift or overspray from nearby spray areas may cause symptoms identical to those of the virus.