Tar spots of maple, caused by species of the Rhytisma fungus, are prominent now, although mostly not as severe as in wetter summers. There are two different tar spot diseases of maple, one affecting silver and red maples, caused by Rhytisma acerinum, resulting in dense, tarry spots that truly reflect the “tar” spot name (first picture below).
On Norway maples the Rhytisma punctatum fungus causes many mini-spots that coalesce into larger spots surrounded by leaf browning, an inch in size or larger. These spots start with the symptoms of pale yellow areas earlier in the summer, with pin-size signs of the fungus evident and coalescing and quite prominent now, with attendant leaf browning and target-like appearance on the undersides of leaves, creating quite an image (final picture below). Leaf drop may occur.
Often, when I paraphrase a quote from Cornell’s Diseases of Trees and Shrubs that: ”Tar spot of maple is one of the most spectacular – and least important – diseases of maples”, I get a few groans, because of course, disease is in the eye of the beholder. Nevertheless, in terms of plant health, even with considerable leaf drop on Norway maple in late July and August in some years, this is not a serious maple disease. And, to allay any fears of customers or clients, these Rhytisma fungi do not spread to other types of plants. The fungus overwinters in fallen leaves, so if there is a concern, raking up the leaves (followed by removal from the landscape or composting) can be effective to limit tar spot incidence.