Question
I bought an autumn blaze maple tree 2 years ago.  It is growing ok but this year it has red spots in the leaves. They are like hard bumps on most of the leaves.  What is this, a disease or what?

Answer
These are leaf spot caused by a fungus. I am going to guess it is Phyllostict leaf spot. A leaf spot that infects late in the growing season causes very little damage except maybe the leaves will fall earlier than normal. If you want to control the fungus the leaf surface would need to be sprayed with a fungicide in the spring. A large tree would be difficult to spray and in my opinion not worth the effort since the damage to the health of the tree is slight.
Phyllosticta Leaf Spot of Maple. Pathogen: Phyllosticta minima

Like many fungal leaf spots, this disease affects a number of maple species, most prominently Amur, Japanese, red and silver maple. The disease causes little damage because the infection is localized. Spots are roughly circular and develop into tannish spots with purple to red borders. Later in the season the spots often contain black fruiting bodies of the fungus arranged in rings inside the lesion. Although this disease is quite noticeable in the landscape, especially on silver and red maples, and causes concern among homeowners, damage is minimal and fungicides are rarely necessary. If fungicides are required use a labeled product containing mancozeb or chlorothalonil.  

It could be a leaf gall insect--a tiny wasp that lays its eggs on the leaf and the cells of the leaf form a gall over the eggs.

Really no need to treat either of these -fungi leaf spot or gall leaf spot. The tree may loose a little growth and may drop its leaves a little sooner than normal but the overall health of the tree is not effected. I would not worry about these leaf spots.