 Tar Spot on a Norway Maple tree
(HRM 2007).
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Maple Tar Spot
What is it?
Tar spot is a distinctive fungal disease that affects red maple, sugar maple, silver maple, Manitoba maple, and Norway maple (including the red-leaved varieties). Tar spot can affect other trees as well.
Will it kill my tree?
No. Although the spots are unsightly, the symptoms of tar spot disease develop late in the growing season, so it has a very minor effect on tree health. Leaves that have a lot of tar spot on them may drop earlier than others.
Where did tar spot come from ?
There are two types of fungus that cause tar spot; one is a type that has always been in North America. Researchers suspect that the other type of tar spot fungus was introduced to North America from Europe in the 1930s.
Tar Spot Lifecycle
Fallen leaves with tar spot will overwinter. In the spring when the warmer weather arrives, the fungus spores in the black spot activate inside the leaf. With spring rains, or an extended period of damp weather, slits will open in the tar spot that release thin, sticky fungus spores. These fungus spores infect newly opened maple leaves to complete the cycle.
What can I do to prevent tar spot on my tree?
The best management practice in a home garden situation is to rake up affected leaves in the fall and compost them. Leaves can be placed in your green bin, or bagged for collection on your regular green bin pickup day.
Web Resources
Do you want to find out more? Here are some resources we used to write this factsheet.
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