Early Blight is caused by a fungal pathogen named alternaria solani. It can affect both tomatoes and potatoes. Early blight can be identified on foliage by the small sunken lesions that are brown to black in color. These leaf spots resemble concentric rings much like the rings you see in trees when cut down.
Plants that get early blight can suffer a reduced yield if untreated and even kill the plant entirely. The disease can and will spread to the stems and the fruit if there is no intervention. Heavy rain can contribute to the spread of this disease as it will cause the fungus to splash up on the plant. Some of this can be avoided by watering at the base of the plant and mulching over the soil.
When early blight strikes you must remove the affected leaves to keep the fungus from spreading. Be careful to not touch any part of the plant with the leaf or with the cutters or your hands until you have disinfected them as the fungus will spread through contact. You then must spray with an antifungal spray, I personally only use organic methods so I would mix up a cornmeal juice spray and soak the foliage and soil around the plant with it.
Early blight is treatable if caught early enough but you must remain vigilant in your task of keeping an eye on the signs of its return. Even though it is called early blight it can occur on older plants.