Q. I have a Norway Maple, 2 years old, that’s showing signs of powdery mildew especially on new growth. What do you recommend? Jack Ravenel – Tacoma, WA
A. First I would see if there is a way to improve air circulation around the trees. This will keep the leaf surfaces dry which will inhibit the powdery mildew fungus. Even the moisture from dew is enough to encourage the fungus if it does not dry off quickly. You can get dew when night temperatures are much lower than day temperatures and the water in the air is “squeezed” out onto the leaf or other surfaces. If the tree is small enough you can remove and destroy infected leaves. But since you indicate that powdery mildew is showing up on new growth you may consider spraying with a fungicide. An organic fungicide like Sulfur or Copper would be effective.