The “Red Maple” is likely a popular cultivar of the Norway Maple, Acer platanoides, ‘Crimson King.’” The white spots are probably caused by the fungal disease powdery mildew. It is a very common disease that affects thousands of plant species. Powdery mildew thrives in humid conditions and is worse on shaded plants or plant parts with little air movement such as interior and lower branches. It does not cause significant damage to the health of the tree and does not require management. When a tree is severely infected, the disease may cause leaves to yellow and fall prematurely; however, the disease begins too late in the year to cause significant damage. By August and September, much of the critical photosynthetic food production in the plant leaves has already occurred for the season. The fungi that causes powdery mildew over-winters in plant debris and can be a source for re-infection the following year. Removing infected leaves from the lawn is recommended but do not compost them. The powdery mildew affecting the maple is species specific and will not affect other plants in the landscape unless they are maples.

Sources: University of Minnesota, Penn State and Colorado State Universities.