Q. I’m finding circular spots on the trunks of my trees and shrubs. What is it and will it kill my plants?
M.L., Fort Lauderdale
What you are seeing are lichen and they cause no harm to plants. Lichen is a class of complex organisms that have a symbiotic relationship (a fungus and alga or cyanobacterium). Lichen also commonly grows on rocks. Lichens can be white, green or yellow/orange in color. Some have black specks.
Sometimes seeing a large amount of lichen indicates that there is branch die-back and more light is reaching the trunk or limbs. The die-back can be caused by a root rot disease but the lichen is not the culprit. Another reason for lichen to be more prominent is during natural leaf drop in deciduous plants. Again, more light is reaching the lichen or there are fewer leaves to obstruct your view of these interesting organisms.
Lichens indicate the air quality is good since they are sensitive to air pollution.
Adrian Hunsberger is an entomologist/horticulturist with the UF/IFAS Miami-Dade Extension office. Write to Plant Clinic, 18710 SW 288th St., Homestead, FL 33030; e-mail email@example.com.