Several different fungi 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 resembles 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. Here in several upstate New York communities tar spot on Norway maple is particularly troublesome because of early leaf drop.
The first symptoms of infection by a tar spot fungus usually show up in mid-June as small (less than 1/8-inch diameter), pale yellow spots. The spots enlarge and their yellow color intensifies as the season progresses. On red maple and silver maple, a black spot usually develops in each yellow spot by mid-July to early August. The black spot grows in diameter and thickness until, by late summer, it truly does look like a spot of tar. The surface of the spot may have a pattern of wavy indentations or ripples.
Another form of tar spot affects striped and Norway maples. On these trees 20 to 50 small spots, each no larger than a pin-head, appear in late July or early August. On striped maple, the spots do not enlarge much after they first appear. On Norway maple, however, the spots grow and eventually may even merge to yield a larger black mass up to 1 & 1/2 inches in diameter. The surface may be slightly roughened to smooth, but will not be rippled. The fungus may allow attack on the seeds of maple also.
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, needlelike 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” (containing the 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. Also, the appearance of the disease has become widespread across much of New York State, and if others in a neighborhood setting are not managing the disease on their trees with fungicides or proper sanitation, the act of spraying may be a waste of time and money
