By Tim Johnson
Special to Tribiune Newspapers
November 15, 2012
Q: My Norway maple has lots of black spots on the leaves and they are falling off the tree in large quantity. I am worried that the tree is dying. What is this problem and is there anything that I can do for my tree?
-- Bob Winter, Lake Forest
A: From your description the problem sounds like tar spot, which is caused by several different fungi that can infect the leaves of maples. I am seeing many maples throughout the North Shore with this problem. The disease gets its name from the resemblence of the black spots to tar.
Tar spot alone is generally not serious enough to threaten the health of a tree, although it can be very unsightly when there are many spots. Heavy infections will cause the leaves to drop early, as you see on your tree.
The fungi that cause tar spot overwinter on infected leaves that fall to the ground. Spores produced by the overwintering fungi are carried by wind to a susceptible maple host and infect new leaves as they are unfolding in spring, starting a new disease cycle.
Sanitation is the most important step you can take to deal with tar spot, as with many diseases that affect the foliage of plants. Rake up and dispose of the infected leaves as they fall now and as more fall this autumn. This will reduce the amount of overwintering fungi that can infect trees again in spring. Leaves from infected trees nearby also should be cleaned up.
Applying fungicides to prevent this disease generally is not recommended. If you want to explore options and feasibility of treatment for your particular situation, contact a professional arborist.
If disease problems continue in future years, you may want to consider removing your Norway maple and replacing with a shade tree of a different species. Norway maple is considered an invasive species in the Chicago area, meaning it reproduces so prolifically that its seedlings disrupt the habitat of natural areas such as forest preserves. It also is difficult to grow other plants under Norway maples because of their dense shade and their competitive root system.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe (chicagobotanic.org). Send questions to: Gardening Q&A, Sunday, Chicago Tribune, 435 N. Michigan Ave., Chicago, IL 60611-4041; e-mail to email@example.com.