Tomato Blight: How to Identify and Treat Early Blight in Tomatoes
Tomato blight, in its different forms, is a disease that attacks a plant’s foliage, stems, and even fruit.
Early blight (one form of tomato blight) is caused by a fungus, Alternaria solani, which over-winters in the soil and infected plants. Affected plants underproduce. Leaves may drop, leaving fruit open to sunscald.
Early blight’s Latin name is sometimes confused with a form of tomato rot, alternaria, a different tomato problem altogether. To muddle matters further, early blight is occasionally mistaken for Septoria leaf spot because the two diseases infect tomatoes at the same time.
What does early blight look like?
Photo: University of Minnesota Extension
Dark, concentric spots (brown to black), ¼ - ½” in diameter, form on lower leaves and stems. Early blight is marked by tell-tale rings.
Fruit can also be affected; spots often begin near stem of fruit
Lower leaves turn yellow and drop
When does early blight affect plants?
It's most evident during early- to mid-season
It’s most common in humid weather
It often strikes after a period of heavy rainfall
How do you control and treat early blight?
The best control measure for tomato blight is prevention (see below).
Remove and destroy infected leaves (be sure to wash your hands afterwards).
Treat organically with a copper spray, which you can purchase at the hardware store or home improvement center. Follow label directions. You can apply until the leaves are dripping, once a week and after each rain.
Once blight is present and progresses, it becomes more resistant to biofungicide and fungicide. Treat it as soon as possible and on a schedule.
Treat organically with a biofungicide like Serenade (available as a spray or in a concentrate) to lessen symptoms. Follow label instructions. Or apply a fungicide such as chlorothalonil (sold as Fungonil),
Daconil at the first sign of blight when fruit sets (as a preventative measure) or when conditions indicate a strong potential for it to develop. Follow label directions. Re-apply every 7-10 days or after rain. Other diseases (such as late blight, Septoria leaf spot, and gray leaf spot) can be controlled by these biofungicides and fungicides, so application is multi-purpose.