The first symptoms usually appear on older leaves at the bottom of the plant and consist of small, irregular, dark brown to black, dead spots ranging in size from a pinpoint to 1/2 inch in diameter. As the spots enlarge, the lesion is a characteristic "target-spot" or "bull's eye" appearance that may grow together, causing infected leaves to turn yellow and die. Usually the oldest leaves become infected first, drying up and dropping from the plant as the disease progresses up the main stem.
Early blight is caused by the fungus Alternaria solani. This fungus is universally present in fields where these crops have been grown. This is why a three to four year crop rotation is important. Spores form on infested plant debris at the soil surface, especially under alternating wet and dry conditions such as we have had this summer. They are easily carried by air currents, windblown soil, splashing rain, and irrigation water. It is usually necessary to apply fungicide sprays to fully protect plants from early blight. Applications to tomatoes are usually done 2-3 weeks following emergence or soon after transplanting.
Once the disease is noticed, there is little that can be done. Carefully removing impacted foliage can slow the progression of the disease.
Organic early application options include:
• Copper-based products: Some forms of copper hydroxide are approved for use in organic production. Spray every 7 days or as the label specifies.
• Bacillus subtilis: Serenade, Provides suppression. Biological control product that needs-be applied before disease development. Control may be limited under heavy disease pressure.
An excellent resource to help diagnose your gardening problems is OSU Extension’s Ohio Line - Go to http://ohioline.osu.edu/
Holly Utrata-Halcomb, District Administrator