. Late blight is caused by a fungal oomycete pathogen (Phytophthora infestans) that survives from one season to the next in infected host plant tubers. True to its name, the disease occurs later in the growing season, with symptoms often not appearing until after blossom. It can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants. If left untreated, massive crop loss can occur such as the disease-induced Irish Potato Famine of the 1850s.
HOST PLANTS: Common in potatoes and tomatoes.
SYMPTOMS: Late blight lesions can occur on both leaves and stems. The first appearance of lesions commonly occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. Under humid conditions, delicate, whitish growth (pathogen spores) are produced at the edge of the lesion, particularly on the underside of the leaf. Lesions turn brown when they dry up.Active lesions are often surrounded by a halo of gray-green tissue. Once lesions dry up, the white spore masses will not be visible. To help identify late blight if outdoor conditions are not humid enough for spores to be produced, you can place suspect leaves or stems in a closed container with a damp paper towel. Check the leaves after about 12 hours to see if the delicate, white pathogen sporulation is visible on the tissue at the edge of the lesion. On stems, late blight causes brown, greasy looking lesions that frequently appear first at the junction between the stem and leaf, or at the cluster of leaves at the top of the stem.
FAVORED ENVIRONMENT: 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.
LATE BLIGHT 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.