May 26, 2019


This month of rain, mist, humidity and fog is so miserable that one Maine lobsterman has coined the term “Fogust.” It has spoiled a lot of picnics, ball games and hikes, but one of its worst visible consequences is what it has done to the maple trees.

Many of their leaves are so covered with black spots that they have shriveled up and fallen to the ground. The bare branches of some of the maples look as if it were already late fall.

The obvious questions: What is this ugly blight? How serious is it? Will it kill the trees? And what can we do about it? Maine’s state forest pathologist, Bill Ostrofsky, as usual, is ready with the answers.

The blight is a fungus, he says, commonly known as tar spot, for the round black spots, often with yellow rims around them, that can appear on maple leaves in late August and September. The scientific name is Rhytisma acerinum. Norway maples are the most susceptible. Silver and red maples sometimes get it. Sugar maples almost never do. A damp spell can hasten the onset of the disease and make it worse. That’s what has happened this year.

Serious? Not really, says Mr. Ostrofsky. The maples all would have lost their leaves in six weeks anyhow. They have had almost four months of good sugar production through the action of photosynthesis on the chlorophyll in the green leaves. So don’t worry about the trees dying. But he says this may be a very bad year for fall colors.

What can we do about it? The short answer is not much. The forestry books say to rake up the leaves, compost them, and cover them with dirt. That is supposed to keep spores from rising next summer to infect the new leaves. But Mr. Ostrofsky says it is impossible to get rid of all the spores. If next summer is a wet one, the spots will return.

He explains that the black spots produce spores that are like the seeds of the fungus. They fly out into the air, many of them landing on the branches and twigs, where they can lie dormant for a year or more, resisting winter cold. All it will take is a moist spell next summer or the summer after next for a return of the black spots.

There are commercial fungicides that can wipe out much of the blight, but most gardeners don’t bother with them and just hope for dry summers.

The black spot plague will come and go through the years. Unfortunately, it may ruin this fall’s foliage show. Blame it on “Fogust.”

Have feedback? Want to know more? Send us ideas for follow-up stories.

comments for this post are closed

You may also like