Lichen is harmless to your apple tree since it not a parasite like true moss that feeds on its host's valuable water and nutrients. However, lichen is a sign that your apple tree is not growing as it should. The tree's canopy is allowing light to penetrate the trunk and is therefore creating optimum conditions for lichen to grow. After you successfully kill the lichen on your apple tree, revive your apple tree to full health by proper pruning, watering and fertilizing so the lichen does not return.
Scrub the lichen off your tree with a soft nylon brush. Do not harm the bark on the tree in the process.
Wet down the remaining lichen with a hose. This will help the lichen absorb the treatment you are going to use to kill it.
Apply the lichen with a fungicide that contains copper. Kocide DF is an example of a copper-containing fungicide that kills lichen. Such fungicides are usually not labeled to kill lichen, but they are approved to kill other similar fungi such as ball moss. Follow label instructions and do not spray the apples themselves.
Apply a baking soda and water mixture as an alternative to the copper-containing fungicide. Mix in a spray bottle 1/2 cup of baking soda per gallon of water and spray the lichen.