Maple tar spot very visible this year

It's late fall and newspapers from the northeast to the midwest have carried reports on maple tar spot, a highly visible fungal infection of maple trees. The Guelph Mercury, a Southwestern Ontario daily, warns, "almost every maple tree in southern Ontario and the American North West is infected..." The distant Bangor Daily News confirms this position saying, the disease is also "prevalent in Maine..." The Columbus Dispatch assures its readers that the large, black, circular stains are "unsightly but not sinister." So, what is this blight? Where did it come from? And is it harmful to affected trees? Tar spot is most conspicuous in the fall, when the worst stage of the infection is reached. But appearing so late in the year, the overall health of the trees is rarely affected. Cornell University speculates the fungus Rhystisma acerinum may have arrived in North America from Europe in the late '30s. A related fungus, R. americanum, has always been with us and also produces the tell-tale black patches, but lacking the dramatic visual punch of the large spots produced by the European invader. In truth, the disease infects the leaves in late spring, first showing up as yellow or pale green spots on the surface of the maple leaves. But it is not until the spots turn a conspicuous black that non-gardeners take notice. The fungi survive from season to season on the fallen diseased leaves. The spores are carried by spring breezes from the decaying, infected leaves still on the ground to the young, emerging maple leaves, beginning the cycle anew. The City of Montreal is asking residents to help stem the spread of the leaf-blemishing tar spot by raking up all infected leaves and disposing of them. Composting is not good enough. Tree experts at Cornell University say homeowner composts rarely reach the very high temperatures necessary. Spraying fungicides is ineffective and costly, not to mention harmful to the environment. Rake and destroy or learn to live with the annual tar spots, these seem to be the only two alternatives at the moment.