Maple trees, with their colorful fall foliage, useful hardwood and in the case of sugar maples, sweet sap, are one of the most popular trees in the United States. While some varieties of maple trees are hardier than others, they all can suffer from fungal diseases, which usually show up as spots on the leaves. These diseases range from being purely cosmetic to having the potential to kill the maple tree.
Anthracnose is actually caused by a group of different fungi. It attacks all types of hardwood trees, including maple trees. Cool, wet springs accelerate the growth of the fungus. The first symptom in maple trees is often browning of the leaf buds, which can die before they even open. Opened leaves that are infected will develop purplish spots along the veins, which spread and reach to the edges of the leaves, causing them to drop prematurely.
Once the leaves drop, the fungus overwinters in the soil, arising to infest the tree again the next spring. Over time, this cycle can stunt the growth of the tree. The most effective way to treat Anthracnose is to prevent it entirely with an application of fungicide in the spring.
Eutypella canker is caused by the fungus Eutypella parasitica. This fungus attacks the wood of the tree rather than the leaves, causing cankers (cracked, discolored areas) to form along the branches and even the trunks of maple trees. The fungus usually infects through a wound in a tree, often caused by pruning.
If the cankers girdle (encircle) a branch, it will cause dieback, which means the new growth at the tip of the branch will die first, with the rest of the branch dying back towards the trunk of the tree. Carefully prune away infected branches to control the disease.
Tar spot is an unsightly fungal disease, but it will not permanently damage a maple tree. The fungus causes yellow spots to develop on the leaves, which turn black and gooey during the summer. The spots can be as large as one inch in diameter. Fungicides are not usually recommended, as the disease is not serious--just unsightly.
Verticillium wilt is the most dangerous fungal disease affecting maple trees. The Verticillium fungus, which lives in the soil, can kill a maple in just one growing season. It enters the tree through the roots, traveling through the tree along the sap. Trees infected by the Verticillium fungus will suddenly experience dieback, with the top of the tree or a clump of branches turning brown seemingly overnight. The tree may die a few months later or linger for a few years, dying slowly the whole time. For this reason, and because there is no cure, infected trees are usually destroyed immediately to prevent the spread of the disease.