They range in size from a barely discernible dot to lesions one-fourth inch or more in diameter. Frequently, adjacent lesions coalesce, forming large, chocolate brown irregularly shaped spots. Scab lesions occur on or along the leaflet veins, but can be found between the veins. On nuts, the scab lesions are small black dots which become sunken as they mature. When infection is severe, the entire nut surface is black, development is arrested and the nut drops prematurely or fails to grow in the area of infection.
Pecan varieties vary in their susceptibility to the scab fungus.
Strains of the pecan scab fungus can develop that will infect a resistant variety. As the inoculum from these strains increase, a once resistant variety is increasingly infected by the fungus. Even though a variety possibly will not be resistant throughout its entire life, it is best to plant varieties that are productive and resistant to the scab fungus. Trees should be spaced to allow for air circulation between the trees. This encourages rapid drying of the foliage.