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Understanding 'tar spot' on your maples

By John Woodmansee, Whitley County Extension educator

Many homeowners love maple trees because they are some of the best shade trees for the home landscape.
Maples produce seeds that fall like tiny helicopters from the tree. As loved as these trees are, people become alarmed when the leaves on these maple trees have large black spots that look like they've been splattered with tar.
This is one disease I see more or less of depending on the weather we've had for the year. This year, like last year, I've seen a lot of the disease. It is aptly named, "tar spot."
"Every summer we get questions about black spots on maple leaves that look like tar," said Gail Ruhl, Sr. Plant Disease Diagnostician at Purdue University. "These spots are not actually 'tar' on maple, but are rather a fungal disease known as tar spot."
In a recent posting from Purdue Plant and Pest Diagnostic Laboratory (https://ag.purdue.edu/btny/ppdl), Ruhlsaid tar spots on maples are caused by fungi in the genus Rhytisma. The most common species are Rhytisma acerinum and R. punctatum. Symptoms first appear in late spring or early summer as infected leaves develop light green or yellow-green spots. During mid to late summer, black tar-like raised structures are formed on the upper surface of leaves within the yellow spots. R. acerinum causes larger spots that are 0.5 to 2 cm in diameter; R. punctatum causes many small punctate (minute spots or dots) spots that are smaller (about 1mm in diameter). These small spots are typically clustered in an area about the size of the larger solid spot of R. acerinum. Spots caused by R. punctatum are sometimes called speckled tar spots.
To get a more concrete idea of what these spots look like, they are typically about the size of a dime or larger, and jet black. The speckled tar spot cluster is about the same size, but with tiny jet black spots within thecluster. Spots may grow and eventually coalesce to yield a larger black mass up to 1½ inches in diameter. The fungus may also attack the seeds of maple.
Most of our maples in this area are susceptible, including sugar, red, Norway and silver maples.
Many of the homeowners I have interacted with are simply alarmed at the striking appearance of the large, black spots on their maple leaves, and they wonder if the disease will kill their trees. Ruhl said that although tar spot disease may cause premature defoliation, seldom are they detrimental to the overall health of infected trees. At this point in the year, through photosynthesis, the leaves have nearly completed their annual job of manufacturing carbohydrates for the tree for maintenance and growth. That is why many late-season leaf spots,like tar spot, are usually inconsequential to overall tree health.
Homeowners can best manage tar spot diseases by raking and destroying fallen leaves, since the fungi overwinter on leaves, and may serve as a source of reinfection next year.


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