Early Blight

We cannot be positive because diseases are difficult to pinpoint from photos and we can’t see anything specific here. It might be a fungus known as early blight appearing initially on older leaves as irregular, dark or tan-brown spots that spread as the disease advances eventually developing black rings (like a bulls eye target) and yellowed leaves. The entire leaf may then turn yellow and dry up. Infection occurs in warm, rainy or humid weather and spread by wind and rain. Pluck off the damaged leaves immediately and spray the plants with a product containing Neem for its fungicide and insecticide qualities. Here’s a tool to help confirm: http://aggie-horticulture.tamu.edu/vegetable/problem-solvers/tomato-problem-solver/