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The Homeowners Column
Spotlight on common tomato leaf spot diseases
May 5, 2016
State Master Gardener Coordinator
Last week I discussed the many ways we can lessen tomato diseases. To recap, always buy healthy transplants - no leaf or stem spots. The two most common ways we go wrong in our pursuits for the perfect tomato concern how and where we plant. Our top disease-lessen lesson is to provide good growing conditions with full sun (at least 8 hours a day) and well-drained soil. Add compost at planting time to feed the soil that feeds your plants.
The second disease-lessen lesson is to provide excellent air circulation around plants. Most of the infectious diseases require water on the leaves in order to infect. Water can be due to rain, dew or over-head watering. Always irrigate the soil, not the plants. Except for miniature tomatoes, allow 4-5 feet between plants at planting.
We can lessen leaf spot diseases; however during periods of continued rainfall and high humidity diseases can run rampant. The most common symptom I hear occurs early in the season. Lower leaves turn yellow then brown and die, appearing to die from the bottom up. Two common fungal diseases, Septoria Leaf Spot and Alternaria Early Blight, can cause this symptom.
As the diseases continue more leaves are infected until the plant has few healthy green leaves. The brown leaves often do not fall off and hang like wet brown socks on the stem. Although tomato wilt diseases of verticillium and fusarium can also cause yellow leaves and death of plants, Septoria and Early Blight have associated leaf spots.
Septoria leaf spot is likely the most widespread tomato disease in Illinois. It commonly causes small water-soaked spots which become circular to angular with dark margins and grayish white centers. Septoria causes rapid leaf blight and plants can be infected throughout the season.
Early Blight causes small brown leaf spots with a target-like series of concentric rings in each lesion. From the leaves Early Blight may also infect stems and the tomato fruits causing dark, sunken, leathery areas. This disease is worse when high humidity accompanies temperatures above 75 degrees F. I think we call that "summer in Illinois".
Another fungal disease called Late Blight is not common in Illinois, but can be devastating due to its quick defoliation of plants in as little as two weeks. Late Blight can also infect other members of the tomato family such as potatoes, eggplant and petunia. Common early symptoms on transplants are dark brown lesions on stems or leaves and sometimes white mold will develop. Seedlings quickly die.
If infection occurs on more mature plants, Late Blight symptoms include nickel-sized to larger irregularly shaped, rapidly enlarging, water-soaked, pale green to greenish black lesions which usually start at the margins or tips of the leaves. On fruit, spots are grey/green and look water-soaked and may cover half of the tomato.
Even if gardeners do everything right we can still see some tomato disease. Weekly fungicide sprays of maneb, chlorothalonil or fixed copper starting before or at the onset of disease can be helpful in managing many leaf spots. Many copper-based fungicides are considered organic and may also manage some bacterial tomato diseases. Be sure to read and follow all label directions. Leaf spot diseases are tough to manage once the plants are ravaged by disease. That's when you head to the local farmers' market.
Get an accurate identification of the problem before reaching for fungicides or pruning at ground level. For more information - University of Illinois Extension's Common Problems for Vegetable Crops http://extension.illinois.edu/vegproblems/Problems with plants? Contact your local University of Illinois Extension office http://web.extension.illinois.edu/state/ or UI Plant Clinic at S-417 Turner Hall, 1102 S. Goodwin Ave. in Urbana. Phone: 217-333-0519 http://web.extension.illinois.edu/plantclinic/