Answer: You and everyone you've talked to who are growing tomatoes are having the same problem; tomato plants developing brown spots on the lower foliage are showing symptoms of a fungal disease known as early blight. Early blight is an annual problem for most gardeners. It normally develops into a problem when plants have a heavy fruit set and the area has received rainfall (or you're watering from above). Spores from the fungus are spread to the lower foliage by wind and splashing rain. Leaves must be wet for infection to occur. At 50 degrees F. the leaves must be wet for 12 hours for infection, but at temperatures above 59 degrees F., the length of time for infection is only 3 hours. Leaf spot development is most severe during periods of cloudy days and high humidity. To control the fungus, foliage applications of a fungicide must be made every 7 days until moist conditions (dew included!) no longer exist. Applications should begin when the first fruit is slightly larger than a quarter. Chlorothalonil (Ortho Multipurpose Fungicide or Fertilome Broad Spectrum Fungicide) and mancozeb hydroxide (Kocide 101) are fungicides which can be used on tomatoes for early blight.
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