Dr. Kevin Korus: Laurel wilt poses a threat to Florida-grown avocados
Given its many ports of entry, Florida is often faced with a host of invasive species that threaten our native and agricultural ecosystems. One such invasive species from Asia, the redbay ambrosia beetle (Xyleborus glabratus), was actually introduced to the United States through a port in Georgia. Since its entry into the U.S., however, this beetle has spread to several states in the Southeast.
The ambrosia beetle is a wood-boring insect that creates tunnels and galleries found in both the sapwood and heartwood of trees. These galleries in the heartwood can be diagnostic (although many other wood-boring beetles will create similar tunnels). Other symptoms of infestation include sawdust at the base of the tree and “frass straws” protruding from the perfectly round entry holes. The beetle infestation itself is typically not enough to cause severe damage or death to a tree; however, beetles can have a symbiotic relationship with a plant pathogenic fungus called Raffaelea lauricola. This fungus is released from the beetle and can move inside the tree via the vascular system and water-conducting tissue. The tree senses this infection and sends tylose and gums to block off the pathogen. Unfortunately, this ends up blocking the xylem tissue as well, and causes rapid desiccation (drying out) of the upper canopy.
The disease caused by this fungus is called Laurel wilt, as trees belonging to the family Lauraceae are particularly susceptible. Trees in this family include redbay (Persea borbonia), swamp bay (P. palustris), camphor tree (Cinnamomum camphora) and avocado (P. americana).
Initial symptoms of Laurel wilt include dieback in the upper crown. Leaves at the top of the tree wilt and eventually turn brown. Some leaves will fall off the tree and others may remain. Exposing the vascular system of the tree will reveal long brown or black streaks. If you suspect a tree on your property has Laurel wilt, consider submitting a sample to a Plant Disease Diagnostic Clinic for confirmation.
Systemic fungicides have been used in commercial avocado groves to provide prophylactic control of this disease; however, treatment is too costly for most homeowners and is not a feasible option. Several contact insecticides are effective in controlling the ambrosia beetle, but in order to be efficacious, they must be applied with perfect timing before the beetle enters the tree. Studies have shown that infections are reduced in higher lighting conditions. Therefore, pruning canopies to increase sunlight penetration may help reduce the amount of infection.
Promoting tree health also can reduce the likelihood of infection. Ensure that trees are adequately watered and pruned, and have sufficient fertilizer. Any stress (drought, freeze, flooding, etc.) will cause trees to become more attractive to the ambrosia beetle. Early detection is key to keeping Laurel wilt from becoming prevalent in the environment. Scout trees often for symptoms, and remove and destroy (burn, bury or chip) trees that have been confirmed to have the disease. It is important to remove the entire tree, root system and all, as the fungus can move from tree to tree via root graphs.
This disease would be devastating if it reaches the avocado-producing regions of California, Mexico or Central America. As Florida residents, we have an obligation to be good stewards of the land and do our best to ensure this disease does not move west. If you suspect a tree on your property has Laurel wilt, contact your local Extension agent for further analysis.
For more information about Laurel wilt, visit UF/IFAS EDIS Publication #HS1360: edis.ifas.ufl.edu/publication/HS1360.
— Dr. Kevin Korus is the Agriculture and Natural Resources Extension Agent for UF/IFAS Extension Alachua County. Contact him at kkorus@ufl.edu or 955-2402.
This article originally appeared on The Gainesville Sun: Dr. Kevin Korus: Laurel wilt poses a threat to Florida-grown avocados
