Have you ever seen a crusty growth on the bark of your trees and wondered if it was a disease or was killing your trees? The Texas Plant Disease Diagnostic Laboratory says do not panic. It is not a disease; just lichen, which is not completely a fungus or a bacterium. Lichens are made up of a symbiotic relationship between fungi and green algae or cyanobacteria (or even all three). The alga feeds the fungus through photosynthesis, while the alga receives some food and support from the fungus.
Lichens commonly grow on branches and trunks of certain tree species, and even rocks, fence posts and tombstones. Lichens appear as surface growths that are usually gray or gray green in color. Three forms of lichens exist — crustose (flat type of growth), foliose (leaf-like but with a prostrate growth), and fruticose (bush-like and erect, or hanging growth).
Lichens grow under conditions of high light intensity. Heavy lichen growth is often an indication of poor tree vigor as a result a cultural problem or stress. Lichens are not parasitic to the tree itself, though heavy growth may begin to restrict gaseous exchange from the twigs and can start to block light from reaching the plant’s surface.
According to the Texas Plant Disease Diagnostic Lab, there are currently no chemicals to control lichen growth. The best way to reduce lichen growth is to focus on improving the tree’s vigor and encouraging the growth of a dense canopy, which will reduce light penetration to the surfaces. Fertilization and timely irrigation can be considered strategies for tree vigor improvement. As the canopy density increases, shading will reduce the photosynthetic capacity of the alga, and, over time, the presence of the lichens should be decreased.
Lichen growth present on some of the twig tissue indicates that the tree has experienced past physiological or cultural stresses that have weakened the tree and then the tree canopy, thereby encouraging lichen development.
The Texas Plant Disease Diagnostic Laboratory, located in College Station, started in 1982 and is a Texas A&M AgriLife Extension Service lab managed by the Department of Plant Pathology and Microbiology at Texas A&M University. The mission of the lab is to provide accurate and timely plant disease diagnostic support to Texas A&M AgriLife Extension and Research personnel, Texas Department of Agriculture, the agriculture/green industry, and the people of Texas to protect and secure our plant resources and to promote economic competitiveness.
Currently, the clinic processes, on average, over 2,500 samples a year. For more information on the Texas Disease Diagnostic Lab and to view instructions, submission forms, and fees, visit their website at plantclinic.tamu.edu. You can also follow them on Facebook at “Texas Plant Disease Diagnostic Lab.” Dr. Kevin Ong, director of the lab, does a great job educating the public, the horticulture industry, and my Smith County Master Gardener intern class each year.
Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is author of “Texas Fruit and Vegetable Gardening,” ”Heirloom Gardening in the South,” and “The Rose Rustlers.” You can read his “Greg’s Ramblings” blog at arborgate.com, read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com), and follow him on Facebook at “Greg Grant Gardens.” More science-based lawn and gardening information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu and aggie-horticulture.tamu.edu.