Overview of 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.
Signs and symptoms of Downy leaf spot
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.
Disease cycle of Downy leaf spot
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.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of Downy leaf spot
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.