Issue 14, August 17, 2015

Tar Spots of Maple

Tar spot has been evident on many species of maple this year. Tar spots of maple are caused by several different fungi in the same genus, Rhytisma americanum, R. punctatum, and R. acerinum. Each is known to cause raised, black spots on upper leaf surfaces. Affected leaves appear as if splattered with tar, hence the name Tar Spot.

The first symptom of the disease appears in mid-June as small, pale yellow spots. By mid-July, the yellow spots have expanded and a thick, raised, black stromata forms within the spot. By September, spots truly look like tar and often have a wavy or rippled surface. This disease may cause some early defoliation if severe, but even then it is not believed to significantly impact tree growth and development.

Tar Spot on Maple.

Tar Spot on Silver Maple (Urbana, IL, August 2015)

The fungi that cause tar spots overwinter on infected leaves. In the spring, overwintering fruiting bodies ripen and eject spores. The spores are carried by wind to nearby developing leaves of susceptible hosts. First symptoms appear 1-2 months following the initial infection.

Outbreaks of this disease are relatively infrequent. Trees in moist, sheltered locations, which allow the pathogen to easily survive the winter, may be repeatedly infected. The most effective management practice for a home landscape is to rake and destroy leaves in the fall. This practice will help reduce inoculum capable of causing infections the following spring. Fungicides can be used to protect newly developing leaves, but are not warranted. (Travis Cleveland)

Travis Cleveland

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