These diseases are difficult to control with chemicals as spraying pecan trees usually is not feasible for homeowners because of tree height. SCAB Scab is the most common and damaging pecan disease. It is caused by the fungus Cladosporium caryigenum . The fungus forms small, circular, olive-green to black spots on leaves, leaf petioles and nut shuck tissue. Scab lesions often run together, causing the terminals to die and the catkins to drop.
Pecan tissue is most susceptible when it is young and actively growing.
When scab attacks young expanding leaves and nuts, it stunts and deforms them. By the time the leaf is mature, it is no longer susceptible to scab. The greatest scab damage occurs from nut infection. Early-season infection can significantly reduce yield and crop quality. Fruit, infected early in the season, often drop or crack where scab lesions run together and serve as a point of entry for other pathogenic fungi.