Blight on Tomato Plants


Tomato plants are a favorite of vegetable gardeners. They're easy to grow and there are many disease-resistant varieties. One tomato disease that is still quite common in many tomato varieties is blight. Blight has two varieties, both early and late, both having their own method of control and eradication.

Early Blight

Early tomato blight is a variety of tomato blight caused by the fungus Alternaria solani. This disease is usually characterized in the early stage by a loss of leaves from the lower portion of the plant. Brown or black spots will likely appear on foliage as well, turning into black, concentric rings. Leaves will dry up and yellow before falling off. The tomato of the plant during early blight can become affected as well. The fungus occasionally attacks the stem of the fruit, which causes large sunken areas, making the fruit black and velvety in appearance. This fungus can also affect potato plants.

Late Blight

Late blight is another variety of blight, also caused by a fungus infection. The fungus is different from early blight, being Phytophthora infestans as opposed to Alternaria solani. Late blight is usually contracted during wet, cool seasons. Young and old leaves both at the top and bottom of the plant are affected. Late blight appears as a wet-looking spot on the leaf, forming greenish-black dots rapidly. The fruit of the plant will become rancid-smelling and black. Late blight usually occurs in mid to late August.


For both early and late blight, infection can be caused by wet weather conditions and is exacerbated by soil that drains poorly. Moist conditions encourage fungal growth. Soil that was once affected by the blight fungus may cause further incidence of the disease. Controlling early and late blight requires planting prevention and caring practices that reduce the spread and risk of infection. Disease-free plants should be planted in a new area, away from infected plants as there may still be fungal presence in the soil. Watering at the base of the plant in the morning prevents moisture build up on the foliage of the plant. Crop rotation should be adhered to, preventing fungal buildup. Tomato crops should be rotated every three to four years.

Early Blight Control

Fungicides can help the fight against early blight also. The application of chlorothalonil, maneb and mancozeb fungicides is shown to be effective as treatment, according to Mississippi State University Extension. Fungicides can be mixed into a hand-held spraying tank and sprayed over the plant. Fungicide should be applied as soon as tomato plants appear or are transplanted; repeat every seven to 10 days.

Late Blight Control

Late blight can also be controlled by the application of fungicides, although different varieties are more effective. Application of azoxystrobin at the first sign of disease, repeated every seven to 14 days, will help control spread and kill the fungus. Azoxystrobin should be rotated with another fungicide, such as cyazofamid, for best control.

Keywords: tomato blight, tomato plant disease, blight control

About this Author

Cleveland Van Cecil is a freelancer writer specializing in technology. He has been a freelance writer for three years and has published extensively on, writing articles on subjects as diverse as boat motors and hydroponic gardening. Van Cecil has a Bachelor of Arts in liberal arts from Baldwin-Wallace College.