This article courtesy of Ohio State University Extension Services.
More information at www.bygl.osu.edu.

This is a very exciting time of year. Tomato plants are mostly growing well and have started to set fruit. Some fruit may be just on the verge of early maturity. Other things on tomatoes are growing well, too. The leaf spot diseases of tomatoes have been enjoying the warm, humid weather most of the state has experienced for the past few weeks. Many of these leaf spot diseases can mimic each other in there early stages of development. Hopefully this article will help one to better determine what may be affecting tomato plants. Proper diagnosis will help one select the most appropriate management for each disease.

• Early blight (Alternaria solani) - This is a fungal disease that can infect all above ground parts of the tomato plant. The symptoms usually begin on the leaves and start out as small necrotic (brown) spots that expand rapidly and eventually grow together, usually from the bottom of the plant, up. There is usually a small yellow halo surround the necrotic region. These symptoms eventually form a bull's-eye appearance and cause leaf defoliation. Fruit symptoms include concentric rings of dead tissue, also giving the lesion a bull's-eye appearance, these spots become large, and eventually will cause fruit drop. This disease overwinters on infected plant material in the soil and seed. Use good sanitation practices after harvest and only use seed from a reputable dealer.
• Late blight (Phytophthora infestans) - This is a fungal disease that infects quickly and can infect all above ground parts of the tomato plant. Rapid fungal development is usually enhanced by humidity and rainfall. This disease creates brown, water soaked, or greasy lesions on stems, leaves, and fruit. Management of this disease is very important. Late blight was found in Ohio in late June of this year.
   
• Septoria leaf spot (Septoria lycopersici) - Septoria is a fungal disease of tomatoes that effects the foliage. In some cases the disease can be so severe that it defoliates the entire plant in a growing season causing little to no fruit production. This disease produces lesions that are usually brown, circular, and small with a yellow halo. Fungal fruiting bodies known as pycnidia can be seen usually in the middle of the mature lesion as tiny, black dots. Septoria starts on the lower leaves and works its way up the plant. Sanitation is key in managing this disease.