Apple scab is a disease of Malus trees, such as apple trees, caused by the ascomycete fungus Venturia inaequalis. The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruits. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves are especially susceptible. The disease rarely kills its host, but can significantly reduce fruit yields and fruit quality. Affected fruits are less marketable due to the presence of the black fungal lesions.

Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called "potato blight".) Late blight was a major culprit in the 1840s European, the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae. At first, the spots are gray-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface.