I've received a number of calls and even a few specimens were brought into our office with concern of spots on maple leaves. These leaf specimens have ranged from Norway maple, silver maple, red maple and sugar maple.
The bad news is that this is caused by fungi. These fungi can make the leaves unsightly, disfigured, and even cause premature leaf drop.
The good news is that these are native fungi. They have always been here and always will be. They also rarely cause long term damage to the trees they infect.
Tar spot (Rhytisma acerinum) most commonly affects the leaves of silver and Norway maples. It appears in late spring and early summer and is light green or yellowish at first. As the summer progresses, raised black spots develop within the lighter colored spots.
Phyllosticta (Phyllosticta minima) leaf spot appear as spots and blotches on the leaves. The spots may be yellow, red, gray, brown or black. They may be so small that they are difficult to see or they may be ¾ inch in diameter.
Maple anthracnose is caused by two kinds of fungi, (Discula spp. and Kabatiella apocryta). It causes irregular light brown spots. They can enlarge and form together making a large blotch, which can cause death of the entire leaf. It is possible for this disease to enter into and kill branches if the leaves are heavily infected for successive years.
What all these leaf diseases have in common is they develop during cool, and/or wet, and/or humid weather. Fungal leaf spots usually are not harmful (with the possible exception of anthracnose) and do not require control measures. However, one of the most simple and effective control measures is to rake up and burn the fallen twigs and leaves in the fall.
If a particularly concerned landowner just must protect a particularly valuable maple specimen, then a fungicide containing mancozeb or copper sulfate applied at two intervals, two weeks apart is sufficient.
While these diseases make the leaves unsightly or cause premature leaf drop, they rarely cause long term damage. Raking up and destroying the leaves and twigs in the fall is typically the best control measure.
Contributed by Christopher Jones, Service Forester with the PA DCNR Bureau of Forestry
Pennsylvania Bureau of Forestry foresters are available for assistance to private landowners free of charge. Interested landowners can call