Tomato leaf diseases are very common, especially early blight (Alternaria solani) and septoria leaf spot (Septoria lycopersici). Here is a link to information that may help you determine whether one or both of these are affecting your tomatoes:

Early Blight and Septoria Leaf Spot of Tomatoes

These fungal diseases overwinter primarily on debris from infected plants, including seeds, but spores can also persist on tools, stakes, and other support systems, coldframes, flats, and containers. In the absence of host plants, spores may persist in soil or compost for two or three years.

For best results sterilize containers and use fresh soil annually.

The spores are spread by wind, water, and contact with affected implements, etc.

Control recommendations for these diseases are similar:
1. Remove and destroy affected plants at the end of the growing season (do not compost vines).
2. Avoid growing tomatoes or related plants (potatoes, peppers, eggplant, etc.) in the same place more than once every three to four years.
3. Select disease-resistant varieties.
4. Obtain certified seeds or transplants.
5. Sterilize tools, stakes, etc.
6. Space plants to ensure good air circulation.
7. Avoid overhead watering.
8. Apply mulch after the soil has warmed.
9. Destroy susceptible weeds such as nightshade.
10 If all else fails, apply fungicides. For recommended chemicals, see
Parasitic Diseases of Tomato.