Lichens aren’t bugs or disease

BATON ROUGE — When people notice lichens on trees, shrubs and other plants, they become concerned that they have problems. But they shouldn’t worry, said LSU AgCenter plant scientist Raj Singh.
“Lichens are fascinating creatures,” said Singh, who is often called the “plant doctor” and heads up the AgCenter Plant Diagnostic Center. “They constitute the most asked questions I receive at all the garden shows I attend.”
Lichens are actually small plants that are composed of two different organisms — a fungal partner and a photosynthetic partner living in a symbiotic relationship, he said. The fungus provides the body and shape while the photosynthetic partner — either a green alga or a cyanobacterium — produces the food through photosynthesis.
These small plants are not parasites but rather epiphytes, which grow successfully under different environments and geographical areas ranging from arctic to desert. “They can almost grow on any surface, including roofs and walls of buildings, rocks, shrubs, trees and even iron fence posts,” Singh said.
Lichens have several different growth habits. Some grow flat like a crust; others are filamentous like hair, while others are leafy or branched.
“They come in some of the most vibrant colors, ranging from lime green to bright orange,” Singh said.
Lichens grow slowly and may live a long time. Actively growing lichens are an indication of good air quality; they don’t do well in polluted environments.
So the question is, are lichens dangerous to plants? “The answer is no,” Sing said. “They are not plant pathogens. They use the tree surface as a substrate to grow epiphytically.”
Because they are not parasites, lichens do not derive any nutrients from the host they are growing on.
“Lichens may grow on healthy as well as stressed trees,” Singh said. “They are more noticeable on stressed trees because of the open or thinner canopy. Stressed trees with open canopies allow sunlight to penetrate deep into the canopy, which results in increased growth of lichens.”
Trees and shrubs can be stressed by drought, improper fertilization, compact soils, disease or insect pressure or other poor cultural practices.
Generally, no chemical control is recommended to manage lichens. Homeowners should avoid any biotic stresses including insects, diseases, nematodes and weeds or abiotic stresses including nutrients, drought, water logging and compaction to their trees, Singh said.
Good practices that promote vigorously growing healthy trees and shrubs with dense canopies may reduce lichen growth.