Question
We've been in our house for 8 years, have a 20 foot maple that about 5 years ago started getting black spots.  We were told by a landscaper it's tar spot and to keep the fallen leaves away.  We've tried, the spots come back each year, and the last 2 years the crown of the tree is getting more and more bare, and in the last year we have a lot of lichen on the trunk.  So far this season the leaves have a few dark spots and little dark gold spots--we are in Detroit MI area. Tree doesn't seem any bigger since we've moved here--is this tree dying? If we can save it, how long will it take to come around? Thanks!

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 unacceptabe 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 fertilzie the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diamter scattered around the tree and watered in good. Fertilize just brfore a good rain storm and you will not have to water. This will increase the overall health of the tree and help over come th loss of leaves from the tar spot.