This is a common fungus problem for maple trees. In our area this year (2022) we had a tremendous outbreak of powdery mildew on Big Leaf Maples, as we’re sure nearly everyone saw. Powdery mildew can affect other trees as well. It’s caused by a range of different fungi, different for each species, but the result is the same: white mold on the tree’s stems and leaves. The mold looks like fine, white powder dusted all over the leaves, which is where the name comes from. The good news is while powdery mildew looks alarming and unpleasant, its damage to trees is minimal.
Powdery mildews do not need the same wet conditions that other fungi do. High humidity will promote promote the fungi spore germination, but it does not need wet conditions to infect a maple tree. The white patches we see on maple leaves are the means of producing those spores, which are then carried to other parts of the tree or other trees by the wind. However, don’t get carried away in blaming powdery mildew on other plants as coming from the maple trees. The spores are very specific; spores on maples can colonize only on another maple or similar plant species.
Since powdery mildew overwinters on infected leaves or fallen leaves, part of managing or containing the spread of powdery mildew is gathering up and destroying the fallen leaves in Autumn. The spores are released in the Spring, so it’s best to have the leaves gathered well before then. An additional part of managing/containing the spread of powdery mildew is to apply fungicides during the growing season. Also, using nitrogen fertilizers should be avoided once powdery mildew has made itself known. Fertilizers promote foliage production, which then just provides more surface area for the fungus to prosper upon and spread from. Overhead watering should also be avoided, if possible, when powdery mildew is present in order to further limit moisture and humidity.