Tar spot on maple, caused by the fungus Tarus Spotius Maximilius (just kidding), has been quite conspicuous on maples in many areas of Michigan for several weeks now. This disease has been prominent for at least five years, especially in some areas where the disease was not common in previous years. Presumably, high levels of pathogen spores from infection in previous years accompanied by conducive environmental conditions this past spring initiated high levels of infections. This disease begins in the spring with pinpoint infections on the foliage -- you may have noted tiny, yellow spots in the spring and early summer. (Fig. #3) These small, yellow spots have now enlarged to large, yellow spots up to an inch or so in diameter over the course of the summer and have now become dark (Fig. #4) with the production of fungal mycelium and fructifications. (Fig. #1 & #2)
Because of the incidence, on some trees 100 percent of the foliage is infected with quite a few spots on each leaf, and the conspicuous nature of these spots, many homeowners become rather alarmed. Frequently asked questions include: "Will the tree die?" "Should I cut the tree down?" "Is it an act of terrorism?" and "Should I spray it with something?"
Tar spot is primarily a cosmetic, all-natural, nuisance disease that causes no harm to the tree, primarily because of its late appearance in the season, closely timed with senescence. Nothing can be done at this time except carry on with our normal day to day activity. Some arborists with certain clients who demand "zero tolerance" of insects and diseases, related that they have achieved excellent control of tar spot when they applied typical "anthracnose sprays" (for sycamore, oak, etc.) to maples in the spring. Normally, zero tolerance is not achieved and the disease is of such little significance that the best approach is to do nothing.
Many folair diseases such as tar spot over-winter in fallen leaves. The removal of thorough composting of infected fallen leaves from the landscape may reduce the potential for fungal infections the following spring. Because spores of some disease organisms may be carried great distances, management of some diseases will be impacted to a greater extent than others by this sanitation practice.