I am often asked about a gray-green leafy or crusty growth found on some plants. Many times the homeowner believes the growth in question is killing the plant. These growths are called lichens and in no way responsible for the poor health of any tree or shrub. As a matter of fact, lichens are common on old wooden fence posts.
So, what is lichen? It is simply an unusual plant composed of a fungus and an alga living together in the same body. The alga converts sunlight and the carbon dioxide in the air to food material just as leaves of green plants. The fungus surrounds the alga, protecting it from drying, and lives off the food it provides. I know it sounds like an odd relationship, but is fairly common in nature.
In a damp climate, the alga can live without the fungus. The fungi found in lichens, however, cannot usually live without associating with algae. Only a few fungi can form lichens and a single, specific kind of algae is required by each fungus.
Why are lichens found in association with declining plants? As woody plants loose vigor and decline, the number and size of leaves gradually decreases.
This allows more and more sunlight on the trunks and branches. As soon as enough sunlight is present to support lichen life, these unusual plants will begin to colonize the trunk and branches.
The presence of lichens is often a sign of poor plant vigor, but the lichens are never the cause. They can be brushed off plants with a stiff brush. However, unless the true cause of plant decline is identified and corrected, lichens will reappear.
There is no chemical control for lichens, because none is needed. If plant health is restored by correcting the real cause of decline, leaves will increase in size and number; less sunlight will get to the trunk and limbs; and lichens will gradually disappear.
Another “odd thing” often found in nature is mistletoe. Mistletoe is a flowering plant that parasitizes trees. Although mistletoe does obtain water and minerals from the tree, it does not depend totally on the tree for food. The leathery green leaves of this plant contain chlorophyll and are capable of making their own food from carbon dioxide and water like all other green plants.
Mistletoe does not have a true root system, but forms structures known as haustoria and sinkers, which invade the xylem tissue of the host. These structures extract water and minerals and securely anchor the plant to the host.
Mistletoe produces small white berries. The pulp of these berries is a sticky viscous material. The berries, spread by birds and wind, stick to the host plant on contact.
Once the seed has become firmly attached to the host, it germinates, forming a radicle. This radicle grows along the surface of the host until it locates a suitable point of entry.
Many think of mistletoe as a Christmas decoration. When using it in the house, take care to keep it out of the reach of children and pets. Birds eat the berries without any side effects; however, the berries are extremely toxic to humans.
The stem and leaves are also toxic and are reported to cause skin irritation on contact in some people.
Mistletoe can be controlled by cutting out infected limbs one to two feet below the point of attachment. In a few instances, breaking out the tops of the mistletoe has proven an effective means of control. No chemicals are labeled for control.
For more information contact the Dawson County Extension Office at (706) 265-2442.Clark Beusse is the Dawson County Extension Agent.