Of all the diseased tomatoes that are sent to the University of Maine Cooperative Extension Service’s Plant Diagnostic Clinic, the most common disease is a fungus called Septoria leaf spot. This disease can infect all aboveground parts of the plant except the fruit. If the fruit itself is rotting, you have a different problem.

This disease appears as small spots that are typically about 1/8 inch in diameter. The spots are usually seen first on the lowest leaves about midway through the growing season. As the tomato plants grow, the foliage becomes denser. This reduces the air circulation and when the leaves get wet they stay wet longer. This is especially true for the lowest leaves. When a Septoria spore lands on a wet leaf, it will sprout like a seed and grow into the leaf tissue. When it has grown long enough you will see the leaf spot.

If you look carefully at the spot with a magnifying glass you may see the spore-producing bodies in the dying tissue. Thousands of spores ooze out in long tendrils. When they are hit by a drop of water, they splash onto the surrounding leaves where they will grow and cause new infections. As the season progresses, more and more leaves become infected and the disease works its way up the plant killing leaves as it goes. By the end of the season, all the foliage may have died leaving nothing but green tomatoes behind. This may be fine with you if fried green tomatoes are your favorite food. If not, there are some things you can do to control the disease.

The fungus will winter over on old infected plant tissue. At year’s end, be sure to get rid of the old plants. Turning over the garden at the end of the year will bury any leftover debris, which will rot and not be a problem next year. If you have room, grow your tomatoes in a different location next year where Septoria will not be present. Pick off diseased leaves as you see them during dry weather and grow your tomatoes far apart for good air circulation. Water early in the day so plants will dry quickly and if possible water from below. Finally, there are several fungicides that will effectively prevent the disease. Check the product label to make sure it can be used on tomatoes to control Septoria.

Bruce Watt is the Plant Disease Diagnostician in the University of Maine Cooperative Extension Service’s Pest Management Office in Orono. For information or advice about fungicides, call (800) 287-0279 (in state).