Silver Maple leaves with Tar Spot

Several people have shown me maple leaves with quarter to three-quarter inch (5 to 20mm) diameter black spots. They are caused by a fungus Rhytisma acerinum aptly named Maple Tar Spot. They occur commonly on Silver Maple (Acer saccharinum) and Norway Maple (Acer platanoides). However, they can infect all of our native and introduced maple species.

Silver Maple – L                 Norway Maple – R         Leaves showing Tar Spot

Maple Tar Spot is becoming common in southeastern Michigan. A Michigan State University Diagnostic Facts publication from 2011 states, “Historically, significant outbreaks of tar spot occur infrequently. In Michigan, however, we have seen severely infected trees for the last three years.” This trend is continuing. Individual trees are sometimes infected for a single year.

Stomata are produced on the surface of the spots and release spores into the air. Maple Tar Spot does no real harm to the tree. It overwinters on fallen leaves so raking and removing these leaves will help to control the infection.
Copyright 2015 by Donald Drife

Webpage Michigan Nature Guy
Follow MichiganNatureGuy on Facebook