Lichens are those funny looking grey things that look like wads of paper on your trees. Sometimes homeowners are concerned about the presence of lichens on their trees or shrubbery. Many times I've had people to tell me, "This stuff is killing my shrubs." In fact, in the last couple of weeks I've had several calls from distressed homeowners related to this subject.
Lichens are a combination of algae and fungus which do no harm to trees. The algae and fungus are mutually beneficial to one another and are not parasitic. The algae needs sunlight to carry on photosynthesis, therefore it must be located in a sunny spot. Trees with low vigor and sparse foliage have more "sunny spots" so more lichens may be found on them. Lichens are the indication of an unthrifty tree and not the cause.
Many times when I check unthrifty shrubbery or trees for people, I find that the problem was caused by a nutrient deficiency. If you have trees or shrubs in your yard that are showing these types of symptoms, it would be a good idea to take a soil sample. Then you could correctly fertilize your shrubs which hopefully would correct the problem.
Trees and shrubs located in poorly drained areas may have been damaged during times of frequent rain. On the other side of the coin, another factor that has reduced the vigor of many trees and shrubs over the last several years is drought. One might think that times of adequate moisture could resolve this problem. Trees and shrubs damaged from long-term drought or water-logged soils sometimes never recover. Those that do recover require several seasons to do so. This is one of those scenarios where an ounce of prevention is worth a pound of cure.
One other problem that I see often is lawn mower and weed eater blight. Young trees are extremely susceptible to damage from lawn mower and weed eater nicks. Put a good mulch around young trees so you don't have to get close to them with these tools.