Maple trees dominate many urban landscapes, where large numbers of people live. When these trees become diseased, it tends to attract attention. Although maples don't often contract deadly diseases, the ones they do get detract from their aesthetic appeal. If you know the difference between the serious diseases and the inconsequential ones, you'll be able to care for your maple tree more effectively.
Verticiullum wilt occurs when the soil-borne fungus Verticillum alboatrum invades the roots of a maple tree, causing the vascular tissue to die. As a result, the leaves begin to fall off the tree. In severe cases, entire branches may wilt and die. You can confirm this disease by cutting off an infected branch and looking for green-streaked wood. Controlling Verticillum wilt is difficult, especially in severe cases. Sometimes pruning away the infected branches, and then watering and fertilizing the tree properly will work.
Anthracnose occurs during cool, wet spring weather. The fungus Gloeosporium apocryptum lands on unfolding leaves of the maple causing brown dead spots. As the disease progresses, these spots grow larger, turn purple and finally black. This can inhibit photosynthesis of the plant in severe cases. You cannot control anthracnose during the growing season. Prune off infected branches in the winter and rake up all leaves and branches in the fall. Disposing of these branches in the trash will get rid of some of the fungus spores.
Tar spots occur as a result of the Rhytisma acerinum fungus. The spores land on maple leaves, causing yellow-green spots. These spots turn into raised black tar-like spots that run together. This is not usually life-threatening to the tree but can definitely be ugly. Raking up infected branches and leaves, and carefully disposing of them will decrease tar spot infection.
Powdery mildew is another fungal disease that attacks maple trees. A thin, white film forms over the leaves, turning them yellow and causing them to drop early in some cases. This is only a cosmetic disease, and does not kill the tree. Occasional, light infections don't require chemical treatment, but in heavy infections, wettable sulfur may help, although it sometimes damages the foliage. Follow directions on the package for application instructions.