Question
I have a 25 year old maple tree.  Last year the leaves developed black spots all over them and then fell off.  The trunk of the tree is covered with a light green color.  I had a tree specialist come to check it.  He said he couldn't identify what disease it was, but he treated the tree with fertilizer and antibiotics injected into the bottom of the tree.  This spring the tree still looks green.  The leaves came on and looked good until this last weekend.  Now they once again are covered with big black spots.  I live in the middle of Michigan and the tree gets morning shade and afternoon sun.  I don't want to loose the tree, but I don't know what to do to save it.

Answer
Several different fungi in the genus Rhytisma infect the leaves of maples and cause raised, black spots to form on upper leaf surfaces. The diseases are called "tar spots" because their appearance so closely resemble droplets of tar on leaf surfaces. Tar spot alone is rarely serious enough to threaten the health of trees, but sometimes there can be so many spots that the tree becomes unsightly. Heavy infections can also cause early leaf drop-- a circumstance that causes the greatest consternation to homeowners because lawns are littered and must be raked before autumn officially arrives.

The fungi that cause tar spots overwinter on infected leaves that fall to the ground. The following spring, just as new leaves are unfolding, the fungal tissue in the leaves on the ground ripens. The surfaces of the spots split and minute, needle-like spores escape. The spores are carried about by wind and if they land on new leaves of a susceptible host they may germinate, penetrate the leaf tissue, and start a new disease cycle.

Current research has shown that the tar spot fungus does not cause long term damage to the host. The most effective management practice in a home lawn situation is to rake and destroy leaves in the fall. This will reduce the number of overwintering "spots" (fungal reproductive structures) which can produce spores the following spring. However, where other infected trees are growing nearby, those leaves should also be raked and destroyed. Mulching leaves will suffice to destroy many of the spots before they mature, but the mulch pile should be covered or turned before new leaves begin to emerge in the spring.

Application of fungicides are possible when high levels of infection become unacceptable but control of the disease is difficult. Complete coverage of leaf tissue is needed for success and this can be difficult on mature maples. If fungicides are required, use a labeled product containing mancozeb or triadimefon or copper.

I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Fertilize just before a good rain storm and you will not have to water. This will increase the overall health of the tree and help over come the loss of leaves from the tar spot.  
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Injecting a thin barked trees like maple is not good--it can cause a wound and allow decay fungi in. It is better to use a granular fertilizer than a liquid injected. Injected fungicides do little for the leaf type fungi. But in the end keep the tree growing healthy and leaf spots  will not kill the tree but can make it look bad at times.