I have this grayish-greenish film on the trunk and branches of my tree. Is this a disease?

No, your tree does not have a disease. The growth is what’s called “lichens”. Lichens are a unique form of life because they are actually two separate organisms; a fungus and an algae living together, one dependant on the other. Lichens are small plant-like organisms that grow just about everywhere; soils, tree trunks and branches, rocks and artificial stones, roofs, fences, walls and even under water. Believe it or not, moss is a lichens.

If a homeowner observes a dead tree or dead branch covered with lichens on it, this is a coincidence; the lichens did not cause the branch or the tree to fail. The tree trunk or branch simply provides a porous surface for the lichen to attach. Lichens are often fairly inconspicuous, but in some moist areas, such as the Pacific Northwest, they may form a mat that completely coats branches.

Lichens are some of the most amazing living things on this planet. They first appeared about 400 million years ago so they have been around for a long time. Any structure that has been standing for a reasonable amount of time is likely to be adorned with lichens. Particularly, they are common on older buildings, stonewalls, in graveyards and on most perennial (living more than 2 years) plants, particularly trees.

Lichens can be shades of green, gray, orange and yellow. The real beauty of lichens however is in their intricate shapes and often three-dimensional forms. They appear to be filigreed or that of fine lace work.

Lichens are also of great use to ecologists. Many lichens are very sensitive to pollution in the air and can tell us if the air is clear and clean. Drug companies make antibiotics from lichen substances. Some lichens make nitrogen in the air usable to plants. Lichens are homes for spiders, mites, lice and other insects. Lastly, lichens can be used as a natural dye to color wool.