Symptoms: Found on tomato and potato plants, late blight is caused by the fungus Phytophthora infestans and is common throughout the United States. True to its name, the disease occurs later in the growing season with symptoms often not appearing until after blossom. It first appears on the lower, older leaves as water-soaked, gray-green spots. As the disease matures, these spots darken and a white fungal growth forms on the undersides. Eventually the entire plant will become infected. Crops can be severely damaged.
Unlike other fungal diseases, late blight does not overwinter in the soil or on garden trash. Instead the spores are introduced by infected tubers, transplants or seeds. Wind will also carry the disease from nearby gardens. Warm temperatures (70-80 degrees F.) and wet, humid conditions promote its rapid spread.
Note: Late blight was responsible for the Irish potato famine (1845-1849).
Control: Plant resistant cultivars when available. To avoid late blight, remove volunteers from the garden prior to planting and space plants far enough apart to allow for plenty of air circulation. Water in the early morning hours (avoid overhead irrigation) to give the plants time to dry out during the day. If symptoms are observed, begin applying a copper based fungicide every 7-10 days during wet weather. Organic fungicides will not kill the disease, but prevent the fungal spores from germinating. Destroy all tomato and potato debris after harvest.
Photo Credit: Meg McGrath, Cornell University