We get many calls each year about maple trees with spots on the leaves. These are usually benign and more stressful to the homeowner (tree-owner) than to the tree itself. Most leaf spots are caused by fungal disease and are worse in years with very wet and cool spring weather, which enhances the development of foliar leaf diseases.
The two most common maple leaf spots are anthracnose and maple tar spot. Maple anthracnose is more common on silver maples and red maples. It manifests as irregularly shaped black, necrotic areas between the veins of the leaves. It mainly affects the lower tier of leaves. It can cause shriveling and premature leaf drop of more severely affected leaves. It rarely causes even 5 to 10 % defoliation. If severe leaf loss occurs, an injection of Phosphojet can assist with refoliation. Injection of propiconazole in the fall can prevent anthracnose in the following spring, but is rarely needed or advised.
Maple tar spot mainly affects Norway maples and to a lesser extent, silver maples. It starts as yellow spots in June or July and then the spots enlarge and turn black in August and September. When severe it can cause significant defoliation, however, defoliation is not as stressful to the tree when it occurs late in the year when trees are soon to drop their leaves anyway. When defoliation is severe, an injection of Phosphojet may be warranted. Some people have had some success preventing tar spot with either early spring fungicide sprays or late fall fungicide injections. Those treatments are available, but since the condition is mainly benign and cosmetic, we are rarely asked to apply these measures.