Stopping powdery mildew on maples

About three years ago, I planted two maples?a red one and a green one. The red maple developed a white, dusty mold on its leaves and it has now spread to the green one. Do you know what it is and how can I get rid of it?

?Alda Fenninger, LaSalle, ON

This sounds like powdery mildew, a fungus disease that is hitting a lot of maples, especially the red-foliaged ?Crimson King? Norway maple. Powdery mildew is more unsightly than dangerous to the tree?s health. However, to help stop further infection, do some judicious pruning (after the trees have fully leafed out) to improve air circulation. Boost your trees? health by proper watering and mulching with compost. Most important, pick up all fallen leaves. If they are left on the ground, the fungus spores will get into the soil and reinfect the tree next year.

?Marjorie Harris, Editor at Large