IR-4's Effort to Combat “Laurel Wilt”: A Threat to the Florida Avocado Growers

 — by Michelle Samuel-Foo, IR-4 Southern Region Field Coordinator; Kathryn Homa, IR-4 Research Coordinator, Fungicides; and Jonathan Crane, Professor, University of Florida TREC

An untreated avocado tree in a commercial planting that has succumbed to laurel wilt disease.
Photo by Jonathan Crane

 An aerial view of a typical avocado grove in South Florida quickly reveals the tell-tale sign that evokes trepidation among growers. Large, conspicuous patches of brown leaves among a sea of bright green canopies, that seemingly appear overnight. The culprits behind this phenomenon are redbay ambrosia (RBAB) beetles (Xylebporus glabratus) and their fungal symbiont Raffaelea lauricola. The fungus causes “Laurel Wilt” which is a devastating disease of redbays

Passive infusion
system used in the
Homestead FL trial. Blue
dye is being used here to
for illustrative purposes only.

Photo by Jonathan Crane

(Persea borbonia), avocadoes (Persea Americana) and other members of the Lauraceae family. The tiny beetles (~2mm in length) are exotic wood- borers that are native to South East Asia (India, Japan, Myanmar and Taiwan). Initial detection in the United States was near Savannah Georgia in 2002, in a survey trap. It is believed that the pest was accidently introduced into the US through some type of wood shipping material (e.g. wooden crate or pallet) at an overseas entry port. Accidental introductions of invasive species are unfortunately not uncommon and can be quite expensive to control. The discovery of the beetles did not raise serious concerns at the time as they were not known to cause significant tree mortality in their native ranges. By 2003 however, the beetle and its symbiont fungus were deemed responsible for redbay die-off’s in Georgia and South Carolina. The pest was first discovered on redbays in Florida in 2005 (Duval county) and since that time, laurel wilt has spread as far south as Miami-Dade county where it is now threatening commercial groves and homeowner plantings.

Most Ambrosia beetle species are attracted to stressed, dying or dead trees and plants. The plants may be stressed due to a number of factors such as drought, flooding, freezing temperatures etc. This is in contrast to the redbay ambrosia beetle which attacks healthy trees. Recent data has demonstrated that up to seven ambrosia beetle species are contaminated with the laurel wilt pathogen and two have been confirmed capable of transmitting the disease. Ambrosia beetles make pin-hole sized borings into the sapwood of the host trees creating galleries that become inoculated with the fungal pathogen. The spores eventually germinate and infect the host plant tissue. As the fungus spreads, the flow of water and nutrients in the xylem and phloem of the host tree becomes disrupted causing them to wilt and eventually die. As the fungus grows on the living wood of the trees, feeding on its sapwood, the beetle and its larvae that were hatched from the galleries, feed upon it. Following pupation, adult females carrying the Raffaelea fungus in their specialized mycangia (fungal carrying sacs) disperse and inoculate neighboring trees.

Laurel wilt poses a serious threat to Florida’s avocado industry which covers about 7,400 acres and is estimated to be worth about $13 million annually. At present, recommendations to curtail the spread of the disease involve removal and destruction of infested trees. In an effort to aid in controlling this devastating disease, IR-4 is working closely with researchers at the University of Florida’s Tropical Research and Education center in Homestead, along with cooperators at University of Puerto Rico Mayaguez and University of California Davis, to procure a registration for the fungicide propiconazole (Tilt) to protect the avocado trees.

Layout of the pressurized infusion system is inserted into base of the tree and into its flare roots. The anatomy of the tree facilitates the uptake and spread of the fungicide. Photo by Jonathan Crane

This study is unique in that test material is applied directly into the avocado tree via trunk injections and root flair infusions. In this type of application, the test substance (propiconazole) is introduced systemically into the xylem (water and nutrient conducting tissue) of the tree. The application methods that are being used for the IR-4 study are passive and pressurized infusions via the flair roots (surface roots emerging from the base of the tree) and injections into the trunk using large syringes. With the pressurized infusion system, the test substance is injected into the xylem of the flair roots of the tree using a hand-held pressurized sprayer with a pressure gauge that is attached to vinyl tubing with macro-infusion tees. In the passive infusion system, an IV bag containing the test substance is hung from a branch on the tree, and the vinyl tubing attached to macro-infusion tees are inserted into the xylem of the flair roots. The test substance is then passively absorbed by the tree. To help visualize this, picture an IV treatment that a person would receive at a hospital. Both of these systems are low in cost and reusable although there are advantages and disadvantages to both methods of delivery. The passive system is less expensive to build, but can be slower. Although the pressurized system is faster, the system needs to be monitored for pressure loss during uptake. With both systems, the rate at which the fungicide is taken up depends on weather conditions and the physiological activities going on in the tree including growth and flowering. Applications can take anywhere from 20 minutes to overnight to complete. It should also be noted that the site must be prepared before beginning infusion applications. Debris from the base of the tree must be removed to expose the top section of the flair roots and the orchards need to be continuously irrigated ahead of the applications. Holes into which the macro-infusion tees will be inserted also have to be drilled. The rate of propiconazole being applied is calculated according to the size of the trunk diameter. Macro-infusion tees must be disinfected between infusions conducted on each tree.

The use pattern that is being tested in the IR-4 residue study includes 2 applications at intervals of 90 (± 7) days with pre-harvest intervals (PHI) being proposed at 0 and 7 days. Two trials (one at each field site) were conducted in Homestead, Florida and Riverside, California during 2013. In 2014, trials sites will include Homestead Florida, Juana Diaz, Puerto Rico and Riverside, California.
Currently, the use of the ‘spot treatment’ method is the recommended method used in order to protect avocado trees that are adjacent to avocado trees that are infected with Laurel Wilt disease. This method stops root transmission of the pathogen. Efficacy work conducted in Homestead, Florida by Dr. Jonathan Crane and others has demonstrated that infusion treatments have resulted in mature avocado trees being protected for approximately 18 months from the Laurel Wilt pathogen. When registered, propiconazole treatments will become a key component in controlling the spread of Laurel Wilt, along with maintaining tree health, sanitation, complete removal and destruction of infected trees, and direct control of ambrosia beetles via insecticides.