Downy Leaf Spot
Downy leaf spot, also referred to as white mold or white leaf spot, is caused by the fungus Microstroma juglandis. Downy leaf spot affects various species of walnut and hickory.
As the leaves of infected trees approach full size in spring, the undersides show a white powdery coating, often more concentrated along the leaf veins. This white coating consists of fungal structures and spores. The upper leaf surface usually shows chlorotic (yellow) spots.
The fungus may also cause broom formation (abnormal clustering of twigs) near the ends of branches. The leaves in the brooms are usually stunted and yellowish, and often drop beginning in midsummer. It appears that the fungus grows perennially in twigs and buds.
Downy spot is considered to be economically insignificant. Fungicide control measures are not recommended. If desired, brooms can be pruned to improve the appearance of the tree.