Cutting just below the skin reveals a dark, reddish-brown, dry, corky rot. Computer-based programs are available to track weather conditions and help predict or forecast when the disease may occur. Disease forecasting programs predict when environmental conditions favour disease and recommend when fungicide applications may be needed. As the risk of disease becomes greater, crop monitoring should be intensified. These lesions usually appear first on the lower leaves.
Lesions often begin to develop near the leaf tips or edges, where dew is retained the longest.
During cool, moist weather, these lesions expand rapidly into large, dark brown or black lesions, often appearing greasy. The lesions are not limited by leaf veins, and as new infections occur and existing infections coalesce, entire leaves can become blighted and die within a few days. The lesions may expand down petioles and stems of the plant. Scouting Techniques Late blight, requires a cool, moist environment. If infected leaves are examined in the early morning or during other cool damp weather, a white mildew growth may be seen on the underside of those leaves.