Out! Out! Darned Spot!

The office that I work in is shared by two agencies: the Isabella Conservation District (that I work for) and the Natural Resources Conservation Service (a branch of the US Department of Agriculture).  Because both of these agencies have the word "Conservation" in their names, people are often confused by what we do - many people come into the office thinking they have found a branch of the Michigan Department of Natural Resources.  One of the things that happens frequently is people will bring in plants or insects that they ask us to identify.  Or they bring in a part of a diseased plant (or a photo of the plant) and ask us both for identification of the problem and for a solution. 

My desk is the first one inside the door so I often end up seeing these things first - even if I'm not here they often end up on my desk with a name and phone number asking me to call.

One question that I answer on an annual basis is about this phenomenon:

Tar-spots on Norway Maple

What are those dark spots on the leaves of my maple tree?

Those dark spot are caused by a fungus in the genus Rhytisma.  They commonly appear on maples during the summer months.  Early in the season they will appear as small yellow dots.  As the season progresses and the fungus grows and matures, the dots become wider and darker.  At this stage they are commonly referred to as tar-spots or tar-spotting because of their resemblance to drops of tar dripped on the leaves.

Another Norway Maple leaf with tar-spotting

Will it kill the tree?

No, even though tar-spots are unattractive, they will not kill the tree.  By the time the fungus progresses to the tar-spot stage the leaves have begun to slow down they production of sugars and are preparing to drop from the tree.  In some cases of serious infection, the tree may drop its leaves earlier than it would if it were not infected.  The tree suffers no permanent harm.

Silver Maple with tar-spots

Is there anything that I can do?

This is the part where I have to be the bearer of bad news to concerned landowners.  There really is nothing you can do in the short run to stop tar-spotting once it has begun in your trees.  The only real thing that can be done is to try to stop the cycle and prevent it from recurring.  The fungus matures in the leaves once they fall from the tree and eventually releases millions of spores that can reinfect the tree.  To stop the cycle, you have to remove all fallen leaves from the site.  Even this is not 100% effective as spores will often blow back in from other nearby infected trees.