Question
Our four year old maple tree has yellowish green leaves with small brown spots on top of leaves.  Can you tell me what might be the problem and how to address it?  Would hate to lose this lovely tree!

Thanks!

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.  

Fungicide need to be sprayed on the leaves as they are budding out and then about every couple of weeks for the first month or two. Treating the leaves after the leaves have matured dose not prevent the fungi from growing. I would not worry about these spots. You can fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of diameter scattered around the tree and watered in good. This will help the leaves green up.