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Tar spot of maple 

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Disease name:  Tar spot of maple

Pathogen name:  Three related fungi, Rhytisma acerinum, R. americanum, and R. punctatum

Hosts:  Tar spot of maple is a fungal disease that affects only maples. It causes a great deal of concern for home gardeners due to its appearance. This disease, however, is considered a cosmetic problem, rather than a real health issue for trees. Tar spot is caused by three related fungi, Rhytisma acerinum, R. americanum, and R. punctatum. Symptoms vary slightly from pathogen to pathogen, but all three organisms produce leaf spots that do resemble spots of tar. While tar spot can affect many species of maple, it is commonly reported on Norway, silver, and red maples.

Early symptoms of tar spot start to develop on leaves in early summer, but often go unnoticed as they are fairly inconspicuous. At this stage, the leaf spots are light green to yellow-green. As the yellow color of the spot deepens, small black flecks begin to develop in the spot. It is not until mid to late summer that the spot darkens to resemble tar.

The development of the tar spot varies by the species of pathogen. One species produces tiny black spots that later coalesce into larger black spots that may be 1/3 to 1/2 inch in diameter. These spots will eventually be slightly raised. Another species of the pathogen causes dark spots that may be an inch or more in diameter. These spots are usually more prominently raised and have a textured surface. The third tar spot pathogen causes smaller spots.

Late in the season, tar spot may cause some leaves to drop early. The number is generally not significant and since leaf drop comes late in the season, relatively little harm is done to the host tree. Tar spot often goes unnoticed until autumn, when leaves are being raked.

The tar spot fungi overwinter in the spots of the infected leaves. In spring, the spots produce new spores that can infect the newly emerging leaves of the host tree. The spores are spread by the wind. If moisture and temperature conditions are right, the young leaves can become infected. Although infection occurs in spring, the development of the disease is fairly slow and symptoms may not be noticed for several weeks.

Cultural Management:

Raking and destroying leaves will reduce the number of spores that survive to the next spring.  For this to be truly effective, everyone with an infected maple in the neighborhood should rake and destroy the leaves of their tree. Since tar spot does little long-term harm to the host, the leaves could be used as mulch. Leaves can be composted, but composting may not completely destroy spores, because home compost piles seldom heat up enough.

Chemical management:  

Fungicides are available to treat tar spot proactively in spring. These are seldom recommended since tar spot does very little damage to the host The cost of treatment may outweigh the benefits.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or