One of the most common diseases of home garden tomatoes is early blight. Gardeners usually describe spots, leaf yellowing, and defoliation, beginning at the bottom of plants and moving upwards. Typically, spots contain irregular concentric rings, and are surrounded by irregular zones of yellow tissue. Stem lesions may also be observed. Fruit symptoms are usually seen on the stem end – the lesions often contain concentric rings. Another tomato disease that also causes defoliation and leaf spots, and can be confused with early blight, is Septoria leaf spot. This disease is most severe in rainy weather. It does not cause symptoms on fruit.
There are a couple of management approaches for early blight. These approaches will be helpful for other fungal disease of tomatoes as well.
Rotation – if the garden is big enough, move the tomatoes around from year to year, with at least 2 years between tomatoes in the same location.
Sanitation – Remove diseased leaves from the plant and destroy; at the end of the season, remove and destroy all of the tomato plants. Don’t allow fruit to rot on the ground.
Irrigation – only water plants during the day so that they are not wet going into evening. When free moisture is present on leaves the fungus that causes early blight can germinate and infect the plant. Drip irrigation is preferable, but if not possible, avoid watering the foliage. Lately the irrigation issue has been pretty much overshadowed by the rainy summers, but irrigation management is still useful during dry periods.
Disease resistance: Not a lot available but there are some varieties with partial resistance.
Fungicides: A weekly spray with Daconil or other product containing chlorothalanil will help quite a lot. Spray intervals can be extended to 10-14 days if the weather is very dry and there isn’t much dew in the morning. I always recommend washing fruit (there are a number of products for this purpose) prior to consumption if fungicides are used.