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A Brief Summary of Laurel Wilt Disease in Florida and the Southeastern United States

A Brief Summary of Laurel Wilt Disease in Florida and the Southeastern United States

Recently, there has been considerable interest and research regarding the laurel wilt disease, which affects members of the Lauraceae family, most notably red bay (Persea borbonia) and swamp bay (Persea palustris).  This article attempts to summarize the aspects of this disease that are of particular interest to land owners and land managers of Florida and elsewhere in the southeastern United States.

The Story of the Ambrosia Beetle, a Symbiotic Fungus, and the Disease Called Laurel Wilt

The disease Laurel wilt is spread by a nonindigenous beetle called the Asian red bay ambrosia beetle, Xyleborus glabratus.  This beetle measures only about 2 mm in length and is cigar-shaped and amber-brown to black in color.  This species has significantly less hair on its dorsal surface and is shinier than other species of ambrosia beetles.  The female ambrosia beetle spreads a nonindigenous fungus, Raffaelea lauricola, into the sapwood of a tree by boring pinhole-sized holes into the branches or trunk and either actively or passively depositing spores of the fungus in the tunnels.  The fungal spores are carried by the beetle in specialized pouch-like structures called ‘mycangia’ that are located at the base of each mandible.  Both adult and larval ambrosia beetles feed on the fungus growing in the tunnels.  Larvae are white with an amber-colored head.  Unlike most other species of ambrosia beetle, which attack dead or dying trees, the Asian red bay ambrosia beetle attacks healthy trees.  The native range of the fungus includes India, Japan, Taiwan, Burma, Bangladesh, and Myanmar.  As you probably guessed, the Asian red bay ambrosia beetle is native to Asia, including the same countries that the fungus is native to. 

The exact mechanism that causes death to a tree infected with the fungus and symbiont ambrosia beetle to die is unknown.  In simplified terms, the death of the tree is the result of it over-reacting to the presence of the pathogen.

The ambrosia beetle and associated fungus are thought to have arrived in the United States from Asia in untreated wood (such as wooden pallets) or in logs.  They were first detected in the United States in Port Wentworth near Savannah, Georgia, in May 2002.  The disease has since spread throughout the Southeast, from North Carolina south to Florida and west to Mississippi.

Relative Infestations of Laurel Wilt Disease among Infected States

State

Area of Coverage

Notes

Alabama

A few counties in the southwestern portion of the state

First detected in the state in 2011

Florida

Throughout most of the state

First detected in the state in 2005.  Not yet detected in some counties of the panhandle and in some the southwestern portion of the state

Georgia

Many counties in the southeastern portion of the state

First detected in the state in 2002, in Port Wentworth near Savannah

Mississippi

A few counties in the extreme southern part of the state

First detected in the state in 2009

North Carolina

Six counties in the southeastern portion of the state

First detected in the state in 2011

South Carolina

Many counties in the southern and eastern portions of the state

First detected in the state in 2004

What Tree Species Does Laurel Wilt Infect?

Although the beetle is named Asian red bay ambrosia beetle, it actually infects several other species, including both native trees and introduced trees of importance to the agricultural and ornamental plant industries.  Below is a list of species known to be susceptible to Laurel wilt.

Trees and Shrubs Known or Suspected to be Susceptible to Laurel Wilt Disease

Common Name

Scientific Name

Notes

Avocado

Persea americana

Introduced, important agricultural crop, important also to the ornamental plant trade

Lancewood

Ocotea coriacea

May be less susceptible to the disease than other members of the family, based on preliminary testing

Northern spicebush

Lindera benzoin

Pondspice

Litsea aestivalis

State-listed as endangered in Florida; demonstrated experimentally to be susceptible to the disease

Red bay

Persea borbonia

Sustained significant mortality due to the disease

Sassafrass

Sassafras albidum

Swamp bay
(incl. silk bay)

Persea palustris

Sustained significant mortality due to the disease

Southern spicebush (AKA pondberry)

Lindera melissifolia

State-listed as endangered in Florida; demonstrated experimentally to be susceptible to the disease

Summary of the Biology and Symptoms of Laurel Wilt Disease

The female ambrosia beetle, attracted to the smell of a red bay tree, bores into the branches or trunk of the tree and deposits spores of the fungus in the tunnels.  Initial symptoms are wilting of the leaves.  Often, wilting is seen in all the leaves associated with the distal portion of an infected branch.  More and more leaves begin to wilt over time as the disease progresses.  Mild discoloration may be seen in the sapwood and can escalate to extensive black/brown streaking over time.  Frass tubes, looking like white bent straws sticking out from the bark, begin to appear months later as beetle activity increases.  It can take as little as a week for a tree to die from laurel wilt during the warm summer months.

Both adult ambrosia beetles and their larvae feed on the fungus growing in the tunnels.  It takes some 30 days from the time the eggs hatch to the development of adult ambrosia beetles.  Males are smaller than females, lack wings, and are haploid.  Females are winged and are diploid.  The fungus can remain alive inside a standing dead tree for at least 1 year according to recent research.  The biology of laurel wilt disease remains poorly understood, and there is significant research to be done to understand the mechanisms involved in susceptibility and resistance.

Management and Prevention of Laurel Wilt Disease

It is no longer logistically feasible to eradicate or stop the progression of the disease considering how widely distributed it is in the southeastern United States.  However, one way to slow the spread on a given site is to cut down and chip dead trees killed by the disease and place the wood chips into piles.  The fungus was found to die about 2 days following chipping, and the ambrosia beetle population of the tree was found to be reduced by 99% following chipping.  The chipping will also reduce the wood available for female beetles to reproduce.

Use of the fungicide propiconazole (Alamo®), injected into the tree, was found to be only mildly effective (approximately 60% survivorship) at protecting red bay trees from the disease.  This treatment is expensive and testing for use against Laurel wilt has been limited so far.  Best results are achieved by systemic injection before any symptoms of the disease are observed on the tree and the pruning of any diseased areas following treatment.  Another method of injecting fungicide, developed by Arborjet®, involves delivering smaller amounts of fungicide using microinjectors.  The results of the effectiveness of the Arborjet® method have not been published as of this writing.  Similarly, the results of the effectiveness of applying fungicide to the soil around a tree have yet to be published.  Fungicides should be administered only by a knowledgeable professional or by the homeowner and in accordance with the instructions and mixing rates on the label.

Insecticides are unlikely to be useful at protecting a tree again the ambrosia beetle.  Broadcast spraying would be harmful to the environment and to beneficial insects, is not likely to be effective against the ambrosia beetle, and is therefore strongly discouraged. 

An attempt was made to protect some trees in Volusia County, Florida, by spraying Pinesol® as a way of “hiding” the trees from detection by the ambrosia beetle.  Pinesol® spraying took place at about 6‑ to 10‑week intervals.  However, all treated trees eventually contracted the disease and subsequently died.  It is possible that baits may be developed in the future that may be more attractive to the beetles than are the trees, but at this point in time no compounds have been identified for use as baits.

Anyone can help reduce the spread of the ambrosia beetle and the associated Laurel Wilt disease.  Refrain from moving untreated firewood far distances.  The State of Florida prohibits movement of untreated firewood farther than 50 miles within the state.  When camping, buy only local firewood or use certified firewood rather than bringing your own.  When traveling abroad, do not bring back untreated wood products or raw plant parts (including seeds or fruits).

Laurel wilt disease is one of at least a dozen tree diseases and insect pests within Florida or neighboring states.  Minimizing the movement of untreated wood and firewood can help reduce the spread of insect pests and diseases such as the emerald ash borer (kills ash trees), Asian longhorned beetle (kills maples), oak wilt and bot canker of oaks (kills oaks), spiraling whitefly (kills several native and ornamental trees), walnut twig beetle and thousand-cankers disease (kills walnuts), sudden oak death (kills oaks), and others.  The reader is encouraged to visit the website www.dontmovefirewood.org for more information.  

Sources and Further Reading:

Global Invasive Species Database.  2010.  Global Invasive Species Database, Raffaelea lauricola (fungus) [online database].  Accessed 10/17/2014 at http://www.issg.org/database/species/ecology.asp?si=1549&lang=EN.

Global Invasive Species Database.  2010.  Global Invasive Species Database, Xyleborus glabratus (insect) [online database].  Accessed 10/17/2014 at http://www.issg.org/database/species/ecology.asp?si=1536.

Spence, D. and J. Smith.  2013.  The status of Laurel Wilt.  Palmetto 30(3):4–5, 8–10. 

U.S. Department of Agriculture, Forest Service.  2013.  Laurel Wilt Distribution Map [online resource].  Accessed 10/17/2014 at http://www.fs.fed.us/r8/foresthealth/laurelwilt/dist_map.shtml.

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