- 1). Provide good air circulation for your tomato plants to prevent tomato blight infection. Space tomato plants far enough apart to allow air to circulate between them and dry wet leaves after a rain or watering. Fungal spores need moist or wet conditions; sun and dryness will kill the blight fungus. Don't plant your tomatoes near a wind barrier; it will reduce air circulation around your tomato plants.
- 2). Start to inspect your tomato plants in June for signs of disease. Early blight can occur anytime during the season, while late blight usually appears later in the season. But during very wet weather, late blight can take hold as early as June. Look for dark spots on leaves and stems.
- 3). If you see spots on leaves, identify the disease so you know how to target the fungus. Many of the early- and late-blight symptoms are similar, but there are also distinguishing characteristics. The leaf spots of early blight are circular and 1/2 inch or less in diameter, with concentric rings and yellow halos. The spots of late blight are usually irregular, start near the edge or tip of the leaf and turn from light green to brown or purplish black. Late blight also affects stems, and whole side shoots may turn blackish and die. The infection for both blights usually starts at the lower leaves. Septoria leaf spot is another disease of tomato plants, but can be distinguished from the blights because its lesions are only 1/8 inch wide. Treatments for the Septoria leaf spot fungus are similar to those for early blight. Septoria leaf spot is not as devastating for tomato plants as the blights.
- 4). Remove any leaves with blight as soon as you see them. For a large infection, pull out the whole plant. Dispose of diseased foliage by burying it deep in the ground in another part of your property or by placing the diseased tomato plant in a plastic garbage bag for trash disposal. Wipe any shears or pruners that you use to remove infected parts of tomato plants with isopropyl alcohol when you move from plant to plant.
- 5). Apply fungicide recommended for either early or late blight. Follow directions for application. Late blight is very difficult to kill, and fungicides work best when applied as a protective measure before plants have been affected.
- 6). Water your tomato plants early in the day so that the leaves will dry out by the end of the day. Alternatively, water only at the roots or use drip irrigation.
- 7). Practice sanitation. Remove all tomato-plant debris and roots at the end of the season. Early blight will overwinter on plant debris. Late blight needs living tissue to survive a freezing winter.
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