Symptoms of scab are rather varied; however, it usually appears first as a tiny, water-soaked, and often raised area on the underside of the leaf. These spots enlarge and may become corky, brown in color, and of irregular size and shape. The condition may also appear on the top of the leaf.
Scab is a physiological condition associated with excess moisture or fluctuations of moisture from too high to too low. There is no biological agent associated with this condition and chemical sprays are ineffective. It is believed that improvement of drainage and growing conditions are the best possible controls.