Septoria leaf spot and early blight are two leaf-spot deases on tomatoes that start showing up about this time of year. Both of these diseases are characterized by brown spots on the leaves.
Despite their names, Septoria leaf spot usually appears earlier in the season than early blight. Septoria produces small dark spots whereas the spots made by early blight are much larger and often have a distorted “target” pattern of concentric circles. Heavily infected leaves will eventually turn yellow and drop. Younger leaves are less susceptible than older leaves. As a result you will see the disease generally start at the bottom of the plant and work upward.
Prevention measures to limit these diseases are fairly simple and consist of using mulch and stakes or cages to help keep plants off the ground. Staked or caged tomatoes have better air circulation around the plants which allows foliage to dry quicker. A good layer of mulch helps to prevent water from splashing and carrying disease spores to plant leaves.
However, sometimes even if you use the best cultural practices and exercise diligent prevention measures, these diseases will still develop. If that is the case it may be necessary to utilize a fungicidal control. Check with your local extension agent, garden center or nurseryman for the most appropriate control recommendations.