COLUMN: Black patches on maple leaves not as scary as they look

Yes, it's September ... already! And while we have been busy, busy, busy going about our lives, Nature has been quietly but steadfastly moving through the seasons as well. And now the time is upon us to enjoy the pageantry of the fall colours, especially as seen in the leaves of the sugar maples. While this overall swirl of colours can indeed be dazzling, you may find a few scenes that appear somewhat less in colour intensity, indeed showing as almost black. Such is the impact of a fungus known as Tar Spot. This very accurate name describes the appearance of black blotches on the leaves of maples, willows, and birch trees. Some leaves have a few splotches and others may be 90 per cent covered. When first encountered, these fungi can be quite off-putting as they look exactly like our preconceived notion of what disease should look like: black, ominous, and not at all attractive. There are three species of tar spot fungi in our area — one that thrives on the upper surfaces of the leaves of sugar, red, and silver maples, another that infects willows, and a third that hits birch leaves. All appear as white, then yellow, then as black patches on the leaves, making themselves visibly known in late summer. A fourth species of tar spot fungus is non-native and affects only the equally non-native Norway maple. And other related fungi can be found on corn, also causing deformed and unsightly leaves. While looking ominous, the good news is that tar spot does not mean the imminent end to that otherwise beautiful maple in your yard. Similar to the timing of the appearance of fall webworm nests, the leaves have pretty well wrapped up their function of absorbing sunlight and are in the process of shutting down. Fortunately, tar spot stays in the leaves and does not travel within the sap; once shed, the leaf and its black markings are no longer a threat to the tree. At least until next spring ... The fungi remain all winter on the fallen leaves and survives under layers of other damp leaves. Like all fungi, it reproduces by spores that float in the breezes, and just as the buds are breaking open, and some of these spores will alight and begin setting up shop on the upper surfaces of the new leaves. As the spores can and do travel widely, one infected tree can set off a cascade of other trees in the neighbourhood becoming infected. As the new leaves grow, the fungi is very light in colour and usually escapes detection. Some photosynthesis occurs even on the infected areas, so the tree does not suffer too badly. So the tree carries on, but what might we folks do to avoid an unsightly mess on our front lawn? If the cycle of the fungal spores can be broken, there is a chance that further spread can be somewhat controlled, maybe, kind of. Infected leaves should be raked up and either burned, buried or disposed of in black plastic bags. The concept is that if the infected leaves are removed in late fall or early spring, the spores will not be allowed to complete their air-borne process. Sounds simple, but good luck with that, as your entire neighbourhood has to remove all their fallen leaves with equal vim and enthusiasm. If any infected leaves are over looked, the spores will appear next May and continue spreading. However, if the tree that was covered in these black spots this fall is now in a cleared zone, there may be a chance it will be free of the fungus next year. However, dealing with air-borne spores is a losing option, as witness by the demise of beech trees. Tar spot: unsightly, yes; a threat to the overall health of the maple tree, no. And so, again like the appearance of fall webworm nests on cherry trees, we can take some solace in knowing the tree will survive despite this visual annoyance. Fortunately, the vast majority of the maples in our area are tar spot free (for now) and there are already individual trees showing some nice reds and oranges along the back country roads. David J. Hawke is a freelance writer and naturalist who lives in rural North Simcoe County. He has been continuously creating weekly nature stories since 1989… and thinks that there are still many more topics to cover!