Several types of fungal infections may cause the leaves of tomato plants to turn yellow and to acquire black spots. Selection of disease-resistant varieties of tomatoes and proper maintenance, including watering, may help avoid yellowed and spotted leaves.
Three common fungal diseases contribute to the development of yellow leaves and black spots on tomato plants. They are Fusarium wilt, early blight and Septoria leaf spot. Fusarium wilt causes older leaves to turn yellow and eventually turn black. Leaves then fall off. Early blight becomes more prevalent in hot summers and causes target-like black spots to develop, according to the Colorado State University Extension. Septoria leaf spot disease causes black spots with a white center to develop.
Once diseases like Septoria leaf spot and Fusarium wilt develop, plants are generally lost. Some basic maintenance tips help make tomato plants more resistant to fungal diseases. Water in the early morning at the base of the plant; rotate crops, planting tomatoes in a different location every three to four years; and avoid walking through wet foliage, suggests the Iowa State University. Treat infected plants with fungicides.
Horticulturists have developed plant varieties resistant to Fusarium wilt and other fungal diseases. Look for plant varieties labeled with VFN, VFNA and VFNT, suggests Colorado State University Extension. The lettered codes, the extension website explains, mean "the plants are resistant to Verticillium wilt (V), Fusarium wilt (F), southern root-knot nematode (N), early blight (A), or tobacco (tomato) mosaic virus (T)."
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