I noticed that all the leaves of my maple tree contain white spots. What do I do to get rid of this residue?
Thank-you for contacting Toronto Master Gardeners with your question.
The white spots on the leaves of your maple look like a form of powdery mildew. However, this fungus usually emerges in late summer. Instead, I believe that the spots you have noticed are an early indication of a fungal disease called Black Spot. The disease is caused by the fungus Rhytisma; in this case most likely Rhytisma acerinum which affects Norway maples (Acer platanoides). This disease has become increasingly prevalent in eastern North America over the past several years. Both the fungus and the tree are non-native, but there are other (native) species of Rhytisma that affect our native maples as well. In Norway maples, the fungus begins its annual cycle when spores are expelled from the previous year’s fruiting bodies. The spores are carried on air currents to settle and grow on the new spring leaves. Cool, damp weather is particularly supportive of the growth of the spores. As the season progresses, the small white dots will enlarge and darken. In the case of Norway Mapless, the black spots can take up most of the leaf by autumn. If the photosynthetic capability of the leaves is sufficiently reduced, the leaves may drop early. All of this sounds and looks very serious, but in fact, black spot is almost never life-threatening to the tree. It cannot be managed with fungicides because they would have to be applied before there is any visible evidence of the disease. The size of a mature Norway also makes spraying untenable. Instead, the recommended practice to reduce the incidence of black spot the following year by clearing away fallen leaves carefully since they harbour the fruiting bodies that will start the infection process all over again next spring.
Here are links to some useful resources: a general description of tar spot from University of Massachusetts , an outline of the Ontario infestation by the Canada Tree Fund , and an article from LEAF on powdery mildew Powdery mildew article with photos.
I hope this is helpful in understanding and dealing with this problem.