Answer: Moss, algae and lichens often grow on the branches and trunks of trees and shrubs where the air is humid. They are a symptom rather than the source of trouble. Lichens are green plants that manufacture their own food; therefore they're not parasitic,and are not taking nutrients from their hosts. Instead, they indicate that your trees aren't receiving enough sunlight and that air circulation is poor. You can discourage lichens from growing on trees by pruning the canopy to open the branches to more sunlight and improved air circulation. You can increase the vigor of your trees by fertilizing during the spring when they're actively growing.
If this doesn't help, a number of fungicides will not only control fungus diseases, but will also kill algae, moss and lichens. A simple lime-sulfur, or a copper fungicide (like Microcop), applied according to label directions, will help kill the lichens. They will eventually sluff off with rainfall. This isn't a permanent solution, though, so you'll have to repeat it as the lichens regrow, and it won't improve plant growth
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