One of the most common inquiries we receive at The Greenline, our Master Gardener help line, goes something like this: "There is something growing on my trees, a disease I think. I want to know what to spray on it to stop the growth."

Oh, how we love to spray questionable vegetation whether it is necessary or not. Our usual response at first involves questions. The most accurate answers come after we request samples and/or photos of the "growth" in question. More often than not, the mysterious invaders turn out to be some form of lichen.Simply put, lichen is an unusual organism that is not a single organism but a combination of two organisms. Lichens are composed of fungus and algae. Their relationship is symbiotic. The word symbiotic comes from the Greek root SYN meaning with and BIOSIS, meaning living. Only certain fungi and algae can live together to form a lichen.The fungi are not capable of producing their own food. They are usually parasites or decomposers which is why they help to make compost well! So, in partnering with algae that produce food readily by the process of photosynthesis, they survive together.Lichens can grow on bare rock, dead wood, desert sand and living bark. They are able to shut down to enable them to survive extreme heat, cold temperatures and drought. Given the right conditions, they can even grow on the roofing shingles of your house. There are many species of lichens here in Pennsylvania.Lichens are sometimes eaten by white tailed deer when winter storms confine their browsing for food. Some birds use lichens for nesting material. Lichens provide food for other small mammals.Through research, scientists have discovered a link between good air quality and the presence of lichens. Lichens take in pollutants and various studies have chemically analyzed what toxic particles were in the air. Lichens are extremely vulnerable to air pollution.So, back to the original question about spraying the often bluish green growths on your trees; relax they are lichens. Take a deep breath and enjoy the fresh air!Editor's note: This article was written by Penn State Extension Master Gardener Julie V. Foley. Keep checking the TIMES NEWS Lifestyle page for more gardening articles from the experts.