No, These Weird Things Called Lichens Don't Kill Your Trees
Here's a question Grumpy gets all the time. "My tree is dying! It has all of these greenish-gray patches on the trunks and branches. What is this disease and how can I get rid of it?"
Well, I have some bad news and some good news. The bad news is your tree really is dying. The good news is those greenish-gray patches have nothing to do with it. Yay!!!!
Those patches represent a unique form of life called lichens (pronounced: lie-kens). See, you be liking them already! A lichen consist of two different organisms engaged in a symbiotic relationship -- which is different from marriage in that lichens usually last a very long time and both partners benefit. One partner is a fungus, whose job it is to anchor the lichen to the branch or stone where it's growing. The other is cyanobacteria (formerly known as blue-green algae), that turn sunlight into food for the lichen. They spread by either spores or tiny plant bits that travel on the wind and use branches and trunks only for support.
There are hundreds of different kinds of lichens and they live everywhere on Earth except in North Korea, where there is no light. A very common type, shown at the top, is called a foliose lichen. It looks like a clump of little lettuce leaves. They're a good food source for hungry wildlife. They're also an excellent source for antibiotic compounds used in human medicine.
However, they do NOT kill trees. So why do so many dying trees sport a thick coating of lichens? Because lichens need sunlight to grow. Dying trees with fewer leaves to shade their branches provides lots of that. Something is killing your trees, but the culprit isn't lichens.
So stop hating on lichens. In Grumpy's enlightened opinion, they're very cool. They're also good for your health, because their presence is a sign of clean, healthy air, as lichens don't like dirty, polluted air.
Are you liking your lichens yet? Oh, yeah!!!