(fungus Fusicladium effusum)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 (fungus Mycosphacrella caryigena)
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. As the lesions age, a pale green-yellow spot develops on the upper side of the leaflet. The spots on the
underside of the leaflets become light brown with a somewhat diffuse margin. Downy spot can cause serious, premature defoliation. The cultivars Stuart and Moneymaker are very susceptible. The downy spot fungus overwinters on
infected leaves.
Powdery Mildew (fungus Microsphacra alni)
Powdery mildew occurs on the green shucks and occasionally leaves of most pecan varieties. Leaf infection develops as pale
green-yellow areas covered with a white-gray, powdery growth. This white-gray dusty growth is also seen on infected shucks. Powdery mildew is most common in hot, humid weather. The cultivar Pabst is very susceptible.
Brown Leaf Spot (fungus Cercospora fusca)
Brown leaf spot occurs on mature pecan leaflets. Diseased areas appear as reddish-brown, small more or less circular lesions. As the lesions develop, they may become more
irregular in shape and cross over veins in the leaflet. Brown spot is seldom a problem in properly sprayed groves, but can cause some premature defoliation in poorly sprayed groves or groves where a fungicide program is terminated
too soon in the summer.
Zonate Leaf Spot (fungus Cristulariella pyramidalis)
Zonate leaf spot occurs only on the mature pecan leaflets. Small gray-brown spots appear on the pecan leaf. These
rapidly enlarge and take on a target-like appearance. The disease develops very suddenly during or soon after prolonged wet periods. The disease spreads throughout the tree very quickly and rapid defoliation soon follows infection.
