This is almost certainly lichen. Lichens are often found on tree trunks, branches and twigs as the bark provides a stable place for lichens to reside and collect sunlight, rainwater and nutrients from the air. They grow on healthy trees, as well as stressed or otherwise unhealthy ones. Homeowners are sometimes concerned about the appearance of these colorful organisms on the bark of their trees or shrubs, and may incorrectly identify them as the cause of plant diseases.

Lichens do not harm trees and shrubs, but it might be a signal that the tree is otherwise stressed by a disease or insect, or is simply aging. They are more apt to appear on trees that are growing slowly so if your tree is already mature, that would be a normal occurrence. If the tree looks healthy otherwise – fully leafed out with no spots, curling, browning, etc on the leaves or fruits – your plants are likely fine. Lichens also thrive in conditions where the air is freer of pollution, and are sometimes used by scientists as an indicator of air quality.

If you feel the need to remove the lichen on your plants, try gently scrubbing the bark of the tree with a soapy solution. Since lichen on tree bark is only lightly attached, it should come off easily. Avoid scrubbing too hard, as this can damage the bark of the tree which will open the tree to disease or pests.

An even better solution is to change the environment where the tree lichens are growing. Lichens thrive in cool, partly sunny, moist locations, so thinning out tree branches overhead to allow more sun and air flow will help. Also avoid routinely using a sprinkler in the area where the lichen is growing, since this will promote lichen growth. Soil under a tree that does not drain well will also contribute to a moist atmosphere, so keep your soil uncompacted by regularly amending with compost.

More info about lichens: https://njaes.rutgers.edu/pubs/fs1205/

Courtesy of NYBG Plant Information Service