Our old silver maple has dark spots on all its leaves this year do you know what is causing it? Is it a disease?
I think your silver maple has a fungal disease called tar spot. Tar spot is a foliar disease: the spores do not affect other parts of maple trees. The fungi responsible for tar spot disease survive the winter on fallen leaves. If these leaves remain in the yard, fungal spores are spread to maple trees in the spring. Spores are produced in the remaining fallen leaves and are carried by air currents to newly emerging maple leaves. By late summer, the spots will take on a black, tar-like appearance. While tar spot is unattractive, the good news is that this fungal disease doesn’t injure the maple tree itself. To avoid the spread of fungal spores, rake up all the affected leaves this fall and bag the leaves for city collection. Do not add these leaves to your home compost pile if you have one. If you remove the infected leaves from the area, you reduce the chances of the tree being infected the following year. It may take a few years of diligently raking up the fallen leaves for the spots to completely disappear.
Michigan State University Extension Service has a good Fact Sheet on Maple Tar Spot Disease. To check it out, click on the following link.