Downy Leaf Spot

Need to know: 

  • A white powdery coating is present on the underside of infected leaves. 
  • Broom formation, or abnormal clustering of twigs, near the ends of branches may occur, often making the twigs stunted and yellowish. 
  • Beginning in midsummer the twigs of the broom formation drop. 
  • Brooms can be pruned to improve appearance of the tree.  

Overview of Downy leaf spot 

Image of downy leaf spot
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.

Symptoms of Downy leaf spot

The upper leaf surface usually shows chlorotic (yellow) spots. Symptoms appear directly above fungal signs present on lower leaf surface. 

Signs 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. 

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.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.

Authors: 

Lina Rodriguez Salamanca Extension Plant Pathologist and Diagnostician

Dr. Lina Rodriguez-Salamanca is a diagnostician and extension plant pathologist with the Iowa State University Plant and Insect Diagnostic Clinic  (clinic.ipm.iastate.edu), a member of the National Plant Diagnostic Network (NPDN, ...

Last Reviewed: 
April, 2022
Category: 

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