Is it Early Blight, Late Blight, or Septoria?:
If you are dealing with Early Blight, the tomato leaves will have one or two spots per leaf, approximately ¼ to ½ inch in diameter. Spots have tan centers with concentric rings in them and yellow halos around the edges. The stems will develop dark, sunken cankers at or above the soil line, and the fruit will start to show dark, sunken spots on the stem end of fruits.
There is no treatment for early blight. Once you discover it in your garden, the best course of action is to pull up and dispose of the plants in the trash -- do not compost or burn blighted plants. Next year, plant your tomatoes in a different area of the garden.
If your tomatoes have late blight, you will see spots that start out pale green, usually near the edges of tips of foliage, and turn brown to purplish-black. In humid conditions, a fuzzy mold appears on the undersides of leaves. The stems will develop black and brown spots that eventually spread. Entire vines can be killed very quickly in periods of high humidity. Fruit damage in the form of dark, leather patches that appear on the top and sides of the green fruit is another sign of late blight.
As with early blight, the method for dealing with late blight is to pull and destroy the plants, look for resistant varieties, and plant in a different part of the garden next year.
Septoria Leaf Spot:
Septoria leaf spot is aptly named, because the primary symptom of the disease is that numerous brown spots appear on the leaves, approximately 1/16 to 1/8 inch in diameter. The spots lack a yellow halo, and, upon close inspection, have black specks in the center. Unlike the above two blights, there is no stem or fruit damage if you are dealing with Septoria.
If your tomatoes are infected, they will keep producing fruit. Simply keep removing infected foliage as you see it, disinfecting your pruning tools when moving from plant to plant. Plant your tomatoes in a different part of the garden next year.