Often, I get the inquiry about that “green and grey fungus that is killing my tree.” At this time of the year when most leaves are off trees and shrubs, these growths are easier to see.
The “fungus” referred to is probably lichen.
Lichens are often blamed for the decline and death of numerous shrubs and trees in landscapes. That’s not too surprising because these unusual plant-like organisms are commonly seen on the exposed limbs and trunks of declining or dead shrubs and trees.
But look carefully around the landscape or other locations and you’ll find them on fence posts, rocks and even gravestones.
These often inconspicuous, hardy, and adaptive plants are a combination of a fungus and an alga. This union or symbiosis produces a long-lived organism that does not look like either the fungal or algal partners. Both partners contribute to the growth of the lichen. The alga uses photosynthesis, like other plants, to produce food while the fungus supplies water and essential minerals and produces a structure that protects the alga from extreme environmental conditions.
Lichens colonize a wide range of exposed surfaces of limbs, stumps, fence posts, soil, rocks, and other living and nonliving objects. They are firmly attached to these hard surfaces.
They are most numerous on limbs and trunks of large mature trees and shrubs in full sun, particularly those plants with badly thinned canopies. Most lichens will not thrive on heavily shaded twigs and branches of healthy woody plants.
Few lichens are found in areas with high levels of ozone, sulfur dioxide, acid rain, and other common air pollutants. As such, lichens are an indicator of good air quality.
Truly, lichens rarely have anything to do with causing poor growth or death of shrubs and trees. Instead their appearance is often related to damage from environmental stress or poor management. Exposed limbs on damaged plants simply give lichens access to the sun they need for growth with little competition.
One way to think of a lichen infestation is as the engine light on your car’s dashboard. When the engine light comes on, we know that is an indicator of what is going on in the engine. We wouldn’t expect a mechanic to fix our dashboard; rather he will fix the engine so the indicator light is not on.
Lichen is the dashboard indicator light that your tree or shrub is having problems.
Good plant vigor is the best defense against lichens. Heavy infestations of lichens are most common on shrubs and trees in declining or poor health and certainly common on dead limbs. Following the right location, recommended plant establishment, watering, and fertility practices will promote the development of a thick leaf canopy, which will inhibit lichen growth on twigs and limbs. Better growing conditions and soil fertility may stimulate new plant growth and ultimately suppress lichen growth.
Light pruning of affected limbs will remove some lichens and stimulate new shoot growth which may help shade out the remaining lichens. Trees and shrubs in extremely poor condition often will not respond to better care and should be replaced.
Presently, no pesticides are registered for the control of lichens commonly found on the twigs and branches of shrubs and trees. If you want to spray with a fungicide, you may reduce their numbers, but you are not addressing the real cause of them- poor tree health.