Septoria Leaf Spot on Tomatoes
Circular spots on the lower leaves of tomatoes may mean your tomatoes are infested with septoria leaf spot.
The small spots of septoria leaf spot are tan or gray with dark brown margins. As the disease progresses the leaves may turn yellow, then brown and die. The disease progresses up the plant, but seldom infects the fruit.
Infected plants usually survive, but with very few leaves and fruit. This loss of leaves will result in a smaller harvest.
Remove diseased leaves as soon as they are found to reduce the spread of the disease. Then do a thorough cleanup in fall.
Mulch the soil to help keep soil borne fungal spores off the plant. Properly space and stake or tower the plants for better air circulation and fewer diseases.
Avoid overhead watering by using soaker hoses or drip irrigation. And rotate your planting locations each year if space allows.
A bit more information: As a last resort, you may choose to use a fungicide. Select one labeled for food crops and apply at the first sign of the disease. Repeat applications are needed in wet seasons. Be sure to read and follow all label directions carefully.