Lichens often get a “bad rap.” They are a common “mistaken identity” and blamed for tree and plant death and decline.
A lichen (pronounced lī-ken) is a relationship between an algae and a fungus, but the fungi do not exist independently and therefore are not capable of causing disease. Lichens have structures called rhizoids, or fungal hyphae, and the rhizoids attach to rocks, bark, branches, and soil. As with Spanish moss, lichens do not parasitize the structures they are living on. They obtain minerals from atmospheric moisture such as rainwater, fog and dew, plant leachates and organic debris.
If your tree appears diseased, consider other contributing factors such as stress, drought, disease, insects and water. For example, if your citrus tree is declining and you see a gummy sap, the problem may be phytophthora, not a lichen.
The UF/IFAS Citrus Research and Education Center provides a website for management and field diagnosis of phytophthora and other citrus diseases. You can access the site at this link: http://www.crec.ifas.ufl.edu/extension/plant_pathology/phytophthora.shtml
Phytophthora is a soil-borne disease. Signs and symptoms of phytophthora include yellow veins, shoot dieback and leaf/fruit drop, trunk damage (often from mechanical equipment), gummosis, sloughing off of roots (root rot) and bark peeling in the crown roots and trunk located near the soil level (foot rot).
Causes of phytophthora or foot rot include poor drainage or soils with hard pans or clay layers, areas with a high water table, overirrigation, tree damage, planting too deep and mulching around the base. Use of mulch under citrus trees can make your tree prone to infection by limiting air circulation. Be sure to maintain a grass and weed free zone beneath your tree.
You can try to scrape off the discolored bark until you reach healthy wood, and apply a copper fungicide to affected areas. Treatment with a copper fungicide may help, but foot rot is very serious and your tree may not recover. In addition, disease occurs when three conditions favor disease development: susceptible host, environment and pest or pathogen. All three need to be present for disease to occur. If you treat with fungicides or replant, and do not address conditions such as poor drainage, the problem may continue to occur.
The UF/IFAS Extension Hillsborough County provides information about citrus diseases. Contact our office if you have questions about how to care for your dooryard citrus.
Extension Agent – Urban Horticulture
Master Gardener Coordinator
UF/IFAS Extension Hillsborough County
5339 County Road 579
Seffner, FL 33584-3334
p: (813) 744-5519 X 54145