While mold and mildew take away from the beauty of trees, they do little harm and generally disappear when dry weather rolls in. Nevertheless, there are things you can do to reduce mold and mildew.
Mold grows on the sugary excretions of certain insects, such as aphids. Generally, these insects excrete this substance on the leaves of trees. However, the substance can drip down onto the bark of the tree as well, allowing mold to grow on the tree's outer skin. Moreover, lichens, composite organisms composed of algae and fungi, grow on the trunks of unhealthy trees and resemble mold.
Mildew, also known as powdery mildew, appears on the leaves or trunks of trees as a white growth. The white growth is actually the result of millions of tiny fungal spores. Trees most commonly affected by mildew include linden, crab apple, catalpa and chokecherry.
Powdery mildew is rarely serious enough to warrant chemical control. Nevertheless, you can get rid of powdery mildew by spraying wettable sulfur at two to four pounds per 100 gallons. Keep in mind that sulfur may harm tender foliage in hot weather. Moreover, sulfur should not be used on walnut trees. As for mold, the goal is to reduce the numbers of honeydew-secreting insects. This can be done by frequently watering the foliage.