Septoria Leaf Spot and Heirloom Tomatoes
by John Yazo
Septoria leaf spot is caused by a fungus known as "Septoria lycopersici", is one of the most common destructive diseases of the tomato in Virginia. This fungus can cause severe leaf spotting and loss of foliage is common with severe infection. If heavy leaf loss happens during wet seasons, it can lead to sunscald of fruit. This can prevent the fruit to mature properly.
Symptoms of this disease are numerous, small, water soaked spots that appear on the lower leaves after the fruit sets. These spots enlarge to a uniform size of about 1/16 to 1/4 inch in diameter. They have dark brown borders and tan colored centers, yellow rings will often surround these spots. When severely infected, the leaves will die and drop off. Septoria leaf spot is easily distinguished from early blight, which is another foliage disease of the tomato, by the uniform, small size of the spots and the lack of concentric rings in the spots. Septoria leaf spot can be confused with bacterial spot of tomato. The presence of the fungus is visible as tiny black specks in the centers of the spots, confirms Septoria leaf spot.
A favorable weather will permit the infection to move up the stem of a plant. This can cause a progressive loss of foliage from the bottom of the plant upward, causing the plants to wither from the bottom up. The loss of foliage will cause a decrease in the size of the fruits and expose the fruit to sunscald. Spotting of the stem and blossoms can also occur.
The fungus from Septoria leaf spot can overwinter in old tomato debris and on wild solanaceous plants like the the ground cherry, nightshade, and jimsonweed. Seeds and transplants that you purchase at your local garden center can also carry the fungus. This disease is favored by moderate temperatures and high rainfall. Spore production is abundant with optimum temperatures of 60°-80°Fahrenheit. The fungus spores are easily spread by wind and rain and infection usually occurs on the lower leaves after the plants begin to set fruit.
Control of this disease can be done by disposal of all infected plant debris, weed control and a good 4 year crop rotation plan.
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