It’s very tempting to claim that this little mess heralds the discovery of a new species of Acer, maybe one we could name “leopard maple” or “polka dot maple.” Alas, the inky blots that appear on these silver maple leaves indicate the presence of tar spot, caused by Rhytisma fungi. Actually, it’s a trio of fungi that decorates maples in this way – Rhytisma acerinum, R. americanum and R. punctatum – and while the fungus can affect many maple species, the spots are most common to Norway, silver and red maples.
It’s fairly obvious why this fungal blight is called “tar spot,” as the mature spots resemble droplets of black tar. But the first signs of infection generally begin in late spring to early summer as small, pale yellow blotches. These then develop a deeper coloration, finally turning black. One species of fungus produces very small black spots that appear to grow and join, finally forming a larger spot that can reach 1/3 to ½ inch in diameter. Often the mature spots are slightly raised and, in some cases, exhibit a sort of finger-print texture.
If the tree affected is either a striped or Norway maple, it’s likely the fungus will manifest as tiny black dots; these may appear on the samaras as well as the leaves. On striped maples, the small spots usually do not enlarge.
Heavy infections may cause premature leaf drop, often as early as August, long before other trees begin to shed their autumn leaves. And this, in fact, may be when the infection is most noticeable. Piles of spotted leaves on the ground are an annoyance, especially if it’s not yet raking season.
As unsightly as the spots may be, they rarely cause health problems. According to Cornell University’s Plant Disease Diagnostic Clinic, current research indicates that tar spot fungi do not cause long-term damage to maples. They do, however, create cosmetic problems and if homeowners are concerned, tar spot can be a headache for landscape and tree care pros.
Fungi overwinter on infected leaves. The most effective management protocol, then, is to rake and destroy all affected leaves in the fall. Spotted leaves that are left on the ground can pose a risk that the fungal reproductive structures will survive the winter to infect new leaves the following spring. Mulching the leaves can destroy the spots before they have a chance to mature, but the mulch pile should be covered or turned before new leaves emerge in spring.
Fungicide application is not necessarily recommended, unless the infection is rampant and the homeowner is insistent. Control is tricky, especially on larger, mature maples, and on those close to neighboring trees that are similarly spotted. Each affected leaf must be completely covered with fungicide in order for the control to be effective. And if fungicide is not applied to trees nearby, the application may simply be a waste of time, effort and money.
The lesson here? It may be unattractive, but tar spot on maple will not cause systemic health problems. If the level of infection is not tolerable, then treatment may be attempted. But the best control is sanitation.