Lichen is a symptom of a plant that is surviving, not thriving. To get rid of it you can do three things.
#1- You need to get your tree or shrub fertilizer.
#2- Spray the lichen with Liquid Copper Fungicide once every 10 days for a month.
#3- You can scrape off the lichen by hand. You do not have to do this third step, but it will speed
up the process of getting your tree happy again.
Below are the are the two products we recommend and use to stop lichen.