There is Early Blight and Late Blight. You will have some Early Blight every year. It is a slow growing blight. If you have Early Blight hold the leaf up to the sun. If you have spots on the leaf that have circles like bulls eye it is early blight. Spray with headline and you can keep it in check. Late Blight is much more aggressive and hard to stop. If you have late blight and it is untreated they will be dead shortly in wet rainy weather.

Late Blight cause the Irish Potato famine