Pecan scab occurs on immature leaves, shucks of developing nuts and current season twigs. Scab lesions begin as small, more or less circular, black spots. Lesions on shucks may run together completely blackening the shuck. Leaf infection seldom causes serious defoliation.
Shuck infection can result in severe losses due to premature nut drop or poor kernel development.
While varieties vary greatly in susceptibility to pecan scab, no variety should be considered completely resistant. The scab fungus overwinters on infected leaf parts, shucks and twigs. Downy spot only occurs on the foliage. Symptoms usually become apparent in late June or early July. The lesions first appear as small, pale green to frosty spots on the lower side of the leaflet.