Lichen thrives in cool, damp climates like that of the Pacific Northwest. When conditions are right, lichen will attach itself to nearly any immobile solid surface including the bark of apple trees. However, although they attach themselves to trees, they do not draw nutrition from them. Lichen are non-parasitic and they photosynthesize their food just like the apple trees they sometimes cover. However, while harmless, lichen are unattractive and many apple tree owners turn to copper sulfate to remove them.
Remove the bulk of the lichen by carefully brushing it off of the tree. Stiffer varieties of lichen can be scraped off with a soft piece of wood. But take care not to brush too hard and damage the bark of the tree. If you notice damage, stop immediately and move on to step 2.
Dilute the copper sulfate according to the manufacturer's instructions (if purchased in concentrate form) and transfer it to a spray bottle.
Spray the lichen with the copper sulfate solution. Spray until the lichen and the underlying bark is quite saturated. This will not only kill existing lichen but prevent it from coming back.
Maintain a regular fertilization schedule. Healthy apple trees have dense canopies that prevent lichen on the bark from photosynthesizing its food. The fertilization schedule necessary to keep your apple tree healthy largely depends on the type of soil that it is grown on, the cultivar and the configuration of your orchard. If you are in doubt about the needs of your apple tree, consult the experts at your local nursery or call a professional.