We get many inquiries about this one from worried growers. “What is this awful disease growing on my apple tree?” they wonder. Well, its not a disease, and it isn’t a worry either…its a lichen. Lichen if you don’t know already, is a symbiotic arrangement. A fungus teams up with an algae or cyanobacteria and they become something else entirely in the process. The fungus supplies out protection and structure, the algae supplies food(sugars) through photsynthesis. Lichen is an epiphyte, that is it lives upon another organism, but not as a parasite.
This from David J. Goerig and James A. Chatfield , Ohio State U. Extension publication:
The short answer is no; lichens do not cause plant damage. The lichen symbiosis is not damaging bark in any direct ways. It does not rob bark of significant amounts of moisture. The fungal symbionts of the lichen do not parasitize living plant cells, and lichens do not appear to be associated with providing entranceways for pathogens into plant tissue. So why do so many people, including many horticulturists, think lichens damage plants? Perhaps it is because when branch decline occurs due to other factors, lichen growth sometimes proliferates. This is due to increased sunlight that penetrates to the bark which favors the algae that are photosynthesizing, resulting in enhanced growth. The lichens did not cause the branch decline, but rather, one of the effects of the plant decline was an increase in lichen growth.
Our experiences :
Lichens usually take hold in earnest when growth becomes meager. I often encounter them on the very oldest apple trees when growth has slowed significantly, even near the tips as new shoots rarely grow more than a few inches. As a matter of course, lichen gets removed from trees in our orchard because we gently rub the coarser bark away from trunks and branches in an attempt to knock off overwintering pests pupae and eggs from crevasses. The light colored lichens can be helpful in reducing sunscald. Since they are a good indicator of clean air (they do very poorly in polluted environments), they probably should be viewed more in a positive light than in a worrisome one.