New Recommendations for Controlling Laurel Wilt
Laurel wilt (LW) disease is caused by the redbay ambrosia beetle and its fungal symbiont Raffaelea lauricola. The beetle was first introduced to the U.S. via infested packing materials from Asia that were brought to Georgia in 2002. By 2012, laurel wilt was detected in a commercial avocado grove located in Homestead, Florida. As a result of R. lauricola transfer by other native and exotic ambrosia beetles (AB), the disease — which can now be found in 11 states including Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee and Texas — poses a threat to California avocado groves.
Dr. Jonathan Crane, professor and associate center director of the University of Florida, Tropical Research and Education Center, Horticultural Sciences Department, and his team of researchers have published a new paper entitled, Recommendations for Control and Mitigation of Laurel Wilt and Ambrosia Beetle Vectors In Commercial Avocado Groves in Florida. Their research continues to lead the way in the search for means of controlling this insect-disease complex, and observations from their latest research follow.
General Observations Concerning LW-AB Complex
Controlling the LW-AB complex is difficult. Laurel wilt infestations can be caused by movement of infested wood, ambrosia beetle movement and rapid root-graft transmission. It is highly virulent — just 39 colony forming units of the pathogen can cause the death of a tree — and the pathogen moves quickly within the xylem of the tree to new locations. In addition, the costs associated with current control tactics can be prohibitive.
In a mature grove, LW can spread very quickly by root grafts or ambrosia beetle infestations. Root transmission can advance to three to six new trees per month. As for the ambrosia beetles, they tend to be most active during dusk and generally fly 10 feet or less from the ground.
Testing of various cultivars indicates that those of Guatemalan x Mexican background are less quickly affected by LW inoculation. No cultivars have been found to be tolerant of the pathogen. Research also indicates that larger trees decline more rapidly upon infection than smaller trees.
Assessment of Current Control Strategies
Research indicates that prophylactic systemic propiconazole (Tilt®) fungicide infusion can protect avocado trees from 12 – 18 months, but it is a costly enterprise and repeated applications can damage the trees. Tilt® fungicide injections are less expensive and can prevent LW outbreaks for about 12 – 24 months. Researchers recommend making the injections prior to an LW outbreak. If that is not possible, inject the healthy-appearing trees and quickly remove the LW-infected trees. Neither of these fungicides are available for use on avocados in California at this time.
Contact insecticides are only effective when ambrosia beetles are on the outside of trees, which isn’t often. Thus, they should only be used on beetles after removing or chipping infected trees.
The use of entomopathogenic fungi (BotaniGard® and Mycotrol®), which infect ambrosia beetles, also is a costly process thus it is recommended this option only be used in later winter through early spring when the beetles are most active.
Cultural Practices and Mitigation
The researchers did determine that pruning trees to increase light levels does suppress AB activity, and it is a more cost-efficient option. Therefore, the researchers recommend implementing and maintaining a pruning program. In addition, top-working or stumping tall trees can help establish better light conditions within the grove that can limit AB populations.
Healthy trees are less susceptible to the LW-AB complex, thus growers should optimize fertilizer and irrigation practices, carefully monitor and care for trees during stressful conditions (drought, freezes, flooding), and implement strategies to prevent root rot and other diseases.
The researchers also recommend frequent grove scouting with an eye for green-leaf wilting. Early detection of laurel wilt is key due to the speed at which the disease can travel through root grafts.
If growers identify trees infected with laurel wilt or infested by ambrosia beetles, they should uproot the trees as quickly as possible to prevent root graft transmission. The entire tree (roots, trunk, limbs) should be chipped/shredded and removed from the grove. Chipped/shredded wood should be sprayed twice with contact insecticide registered for use on avocado.
To protect healthy trees within a one-acre area of an infected/removed tree, contact insecticides should be directly applied to the trunk and major limbs of healthy trees twice within a 14-day interval. To suppress ambrosia beetles during late winter – early spring, two grove-wide applications of BotanicGard®ES or Mycotrol® should occur.
Once the infected trees are removed, new trees can be safely planted to maintain grove productivity. Young trees are not as susceptible to laurel wilt because ambrosia beetles prefer larger, established trees. Young trees also will have more light exposure surrounding them, which depresses ambrosia beetle activity, and movement of the pathogen by root grafting is not an issue for young trees.
Dr. Crane and his team continue to research the LW-AB complex and are currently examining vaccinations to protect avocado trees from the LW pathogen, looking to develop a faster LW diagnostic tool, determining the effect of plant nutrients on AB-LW development, screening scions and rootstocks for tolerance/resistance, examining AB control tactics and suppression, investigating the barrier and bagging method, trying to gain a molecular understanding of the pathogen and researching the economics of the LW epidemic. The Commission will continue to keep California avocado growers informed of the latest updates concerning this insect-disease complex.