Various fungal species cause scab disease in plants. Each species tends to be specific to its host (the plant infected), however, all-cause similar symptoms in plants.
Some of the most common Scab diseases are Apple and Pear Scab. The main symptoms involve dark lesions on the leaves, stems and fruits. The lesions of fruits crust over, forming a black scab, which ultimately turns sooty when the spores are produced.
Scab diseases rarely kill plants, and the damage is mostly aesthetic. However, harmful infections can reduce the quality of crops, and overall growth may be stunted. Trees may weaken and be susceptible to infections by other pathogens.