Pecan scab is caused by the fungus Fusicladium effusum. This fungus attacks the young leaves in the spring and nut shucks throughout the summer. If untreated, leaf infections can become so severe as to defoliate the tree. As they become progressively more severe, nut shuck infections reduce quality (kernel fill and size) of the nut and can result in abortion of the nut. In wet years, many susceptible cultivars lose the entire crop to pecan scab. Commercial growers usually apply several fungicide sprays to control this disease. Because of the large size of the trees, and the number of sprays needed, the best option for the home grower is to plant resistant cultivars.
Pecan leaflet showing symptoms of pecan scab.
Scab fungal infection after about two weeks. The dark brown areas are the fungal reproductive structures. They will protrude up through the leaf surface and produce the next generation of spores. These spores form the dark spots that you see on the leaf and shuck surface.
Pecan nuts showing symptoms of pecan scab.
Germinated spore trying to infect a leaf of a resistant pecan. Note that it hasn't formed the root-like hyphae in the leaf. No infection was established.
Microscopic view of the pecan scab fungus infecting a pecan leaf. The dark football shaped object is the fungal spore. It is lying on the leaf surface, has germinated and penetrated below the leaf cuticle. The root-like blue objects are the fungal hyphae growing between the cuticle and epidermal cell layer. Later, it will penetrate further into the leaf interior.
Transverse section through a scab infected leaf. The gray long structures below the leaf are the fungal fruiting bodies extending below the leaf. This is the feature that would appear black and fuzzy by the naked eye.