The other day, I was out cleaning up leaves in the garden. Not one of my top favorite things to do!
My garden is about 1 1/2 acres and half of it is covered with huge mature trees. In the process of blowing and chopping leaves, I noticed that my maple tree leaves had large black tar like spots. I had seen this before in other gardens, but not in mine. With the trees as tall as they are, it was hard to see the leaves up close.
What is on my leaves looks like someone dropped black tar on the them. Not surprising that this fungal disease is called "Tar Spot". This disease does not harm your tree, but it does make for ugly spotted leaves. Tar Spot disease will over winter on fallen leaves. Make sure that you clean up or mulch your leaves. If you mulch, turn the mulch pile.
The disease starts in the spring during wet weather when the fungus spores are spread by the wind and infect the new leaves as they open. Small yellow-green spots appear that over the summer become larger, thicker and black in color. By late summer the spots are large enough to be noticed. Maples tree such as silver, sugar, red and Norway are infected by this disease. If you have box elder trees on your property, they can also be infected. Spraying for the disease can be a waste of time and money. Raking up and destroying infected leaves is the best line of defense to prevent the disease next year.