If you have oak trees in your lawn or landscape, chances are you have seen the unsightly green fungus-like growth on the trees trunk and bark. This light green growth is not entirely a fungus, but is a lichen, which is a symbiotic relationship of fungus and algae. Although lichens will not harm your tree, when left to grow undisturbed, the lichens will quickly cover the entire tree with a green, mold-like substance.
Use a bristle brush with stiff bristles to scrub away the lichen from the tree. Do not apply pressure to the brush, and you will not damage the oak's bark.
Spray the oak tree with water to soak the lichens. Scrub the tree's bark again, to remove any leftover lichen from the first scrubbing.
Cut away any branches that have lichen growing on them. Lichens begin growing on oaks because the tree's foliage is thin enough to allow light to reach the trunk and branches. Thinning out affected branches will promote new, thicker growth of the tree. Use garden loppers to cut away the branches flush with the main branch or trunk.
Spray the oak tree with a copper-based fungicide to prevent the lichens from reoccurring. You can purchase a copper-based fungicide at a garden specialty store.