Consider the situation: your large Norway maple has big black spots on its leaves throughout the canopy.
This is the first time that you’ve noticed it. If it doesn’t get treated will it die? Or is it too late now anyway?
It’s not as serious as you may have thought.
What you have is a fungal disease called “tar spot.” It is caused by three related fungi called Rhytisma. Symptoms vary from pathogen to pathogen, but all three organisms produce fungal spots that do look like spots of tar. While tar spot can affect many species of maple, it is most commonly reported on Norway, silver, and red maples.
Early symptoms of tar spot begin to develop on leaves in early summer, but often go unnoticed as they are fairly inconspicuous - usually yellow or light green in color. It is not until mid – to late summer that the darker coloration shows up. Late in the season, tar spot may cause some leaves to drop early, which is usually when the problem is discovered.
Black spots on the leaves contain fungal spores that can survive the winter. Raking and destroying leaves will reduce the number of spores that survive to the next spring. Ideally for this to be truly effective, everyone with an infected maple in the neighborhood should rake and destroy the leaves of their trees.
Fungicides are available to treat tar spot proactively in spring, but are seldom recommended since tar spot does very little damage to the tree. The cost of treatment may outweigh the benefits.
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