Fungal diseases designated as scabs characteristically appear as an overgrowth of tissue on leaves, stems, and fruit. The swollen nature of the tissue makes the lesion appear distorted or raised. In some cases of scab, it is difficult to distinguish the raised appearance of the tissue, so this terminology is only somewhat useful to identify the type of disease encountered. Scab diseases affect fruits, vegetables and, to a lesser extent, flowering ornamentals. The most economically important scab disease is apple scab. For this disease, the first symptom is the appearance of yellow to gray areas, notably along the leaf veins. Later, the leaves may turn yellow and then brown with some distortion, curling or cupping. Infected leaves typically defoliate from the tree early in the season. Fruit and stems can also be infected. Repeated leaf loss leads to reduction in fruit production and stress, which leads to other problems. The scab fungus overwinters in fallen leaves and fruit so sanitation practices are important.
Scab diseases on vegetables occur on pumpkin, cucumber, melons, potatoes, squash and beets. Typical of these diseases is the production of a corky lesion or spot on the surface of the fruit and of spots on the leaves. For potato and beets, scab is a disease of the tuber or root that appear as a corky lesion that may cover the entire underground plant part. Managing scab diseases on vegetables is similar to that on fruit. Where underground plant parts are affected, soil preparation practices and crop rotation are important.