The following question was sent to the P&PDL diagnosticians here at Purdue University:

Question: I have several oak trees (species unknown) that have, what I believe to be, a fungus on the trunk of the tree. This "fungus" covers a majority of the tree and is not limited to the north facing side of the tree. The "fungus" is light green, circular in nature and is rather spongy. I am from the Massachusetts area and I have noticed this same affliction on a lot of the oaks in the area, but no one seems to know what it is. Can you help?

lichenAnswer: What you describe sounds like lichens. A lichen is a fungus and an alga growing as an interwoven mass that appears to be a single individual. Lichens grow symbiotically, which means that both the alga and the fungus provides something essential to the other for its survival. Lichens are not harmful to plants. They are merely using the plants or trees as a place to anchor. Lichens occur in a variety of habitats from the Arctic to the Antarctic and all regions in between. One finds them on exposed rocks in the deserts, on solidified laval flows in Hawaii, on frozen substrata in the plar regions, on the bark of trees, and on the leaves of plants. The most important role of lichens, so far as humans are concerned, is as indicators of air pollution. In centers of heavy industrial pollution, no lichens can be found. The lichen population increases gradually with distance from these centers and is thus something of a measure of pollution intensity. So, if lichens are growing in your area, consider yourself lucky.

About 18,000 species of lichens have been described. There are three types of lichens:

1. Crustose (flat, appressed) lichens grow closely appressed to the substrate (ie. rock) or even within its surface.

2. Foliose (leaflike) lichens are flattened like leaves but may not be connected to the substrate at all points.

3. Fruticose (shrublike) lichens have an erect shrublike or filamentous morphology and can be about 10 cm high.

It sounds like you are describing foliose lichens. I hope this information is useful.​