Several diseases affect tomatoes, including early and late blights, septoria leaf spot, verticillium wilt, and fusarium wilt. Varieties differ in their susceptibility to these diseases. A specific diagnosis is difficult without a sample to look at, but try the following to lessen the risk of disease.
Mulch well to prevent the spread of disease spores that live on the soil. Plant in a different place every year to prevent the spread of diseases that live in and on the soil. Provide good airflow around the tomato plants. Most foliar diseases are worse when plants remain wet for longer periods. Choose disease-resistant varieties. (Look for any combination of the letters V, F, N, and/or T on the plant tag.) Consider spraying your plants with a protective fungicide spray. Try growing tomatoes in fresh potting soil in a large container on a sunny deck or in a different area of your yard.