According to Penn State University, blight and mildew are both diseases that affect lilacs. Bacterial blight, ascochyta blight and shoot blight are different varieties of the disease.
Penn State University states that blight is characterized by black or brown leaves, stalks and flowers. When determining which type of blight is affecting lilacs, it is advised to check the color of the plant and the area where wilting occurs. For instance, bacterial blight causes black or brown leaves, shoots and flowers. Ascochyta blight may be recognized by large, circular brown areas on leaves and visible fungus growth. Mildew is a fine, white powder that coats the leaves of the plant. Along with blight and mildew, lilacs are susceptible to invasions by witch's broom, which causes brown growth along the lilac's stalk.