Laidback Gardener Tip of the Day

Learn to Live with Tar Spot

juillet 31Maple tar spot (Rhytisma acerinum) is a disease of the Norway maple (Acer platanoides) that causes pale yellowish spots on the leaf surface in early summer which become, in the month of August, large black spots. In years where the disease is very present, there is also a massive loss of leaves in August, well before normal leaf fall in October. When the spring is rather dry, the disease is still present, but less obvious.

Tar spot is usually considered a cometic disease. Usually, infested maples survive without any significant effect on their long term health. On the other hand, if the disease has begun to become to be a yearly problem in your area, planting new Norway maples is begging for trouble. 

No treatment is really possible, because you would have to spray a very large tree with toxic pesticides several times a year, year after year, which is not only difficult and expensive, but environmentally irresponsible. Apply the Laidback Gardener’s 15 Steps Rule: step back 15 steps and if you do not see the symptoms, it is not worth treating!

Note that other species of tar spot can affect other maples, but they rarely cause significant damage.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

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