Wikipedia
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 leaves1, 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.
Usage examples of "apple scab".
The great majority of the species are minute and come under the definition of microscopic Fungi, and many of these are parasitic, and in many instances prove very destructive to cultivated plants: among such are the species causing Apple Scab, Potato Disease, American Gooseberry Mildew.