Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Raffaelea lauricola
(laurel wilt)

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Datasheet

Raffaelea lauricola (laurel wilt)

Summary

  • Last modified
  • 21 February 2019
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Raffaelea lauricola
  • Preferred Common Name
  • laurel wilt
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Fungi
  •     Phylum: Ascomycota
  •       Subphylum: Pezizomycotina
  •         Class: Sordariomycetes
  • Summary of Invasiveness
  • Laurel wilt is responsible for the death of hundreds of millions of redbay (Persea borbonia sensu lato) trees throughout the southeastern USA, and the disease is also having significant effects on other species...

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Pictures

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PictureTitleCaptionCopyright
Raffaelea lauricola (laurel wilt); symptoms, redbay trees (Persea borbonia) with laurel wilt. Hunting Island, South Carolina, USA. April 2007.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); symptoms, redbay trees (Persea borbonia) with laurel wilt. Hunting Island, South Carolina, USA. April 2007.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); symptoms, redbay trees (Persea borbonia) with laurel wilt. Hunting Island, South Carolina, USA. April 2007.
SymptomsRaffaelea lauricola (laurel wilt); symptoms, redbay trees (Persea borbonia) with laurel wilt. Hunting Island, South Carolina, USA. April 2007.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); healthy redbay tree (Persea borbonia). Horton House, Jekyll Island, Georgia, USA. December 2006.
TitleHealthy redbay tree
CaptionRaffaelea lauricola (laurel wilt); healthy redbay tree (Persea borbonia). Horton House, Jekyll Island, Georgia, USA. December 2006.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); healthy redbay tree (Persea borbonia). Horton House, Jekyll Island, Georgia, USA. December 2006.
Healthy redbay treeRaffaelea lauricola (laurel wilt); healthy redbay tree (Persea borbonia). Horton House, Jekyll Island, Georgia, USA. December 2006.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); 'redbay' (Persea borbonia) redbay in the early stages of laurel wilt. USA. March 2006.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); 'redbay' (Persea borbonia) redbay in the early stages of laurel wilt. USA. March 2006.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); 'redbay' (Persea borbonia) redbay in the early stages of laurel wilt. USA. March 2006.
SymptomsRaffaelea lauricola (laurel wilt); 'redbay' (Persea borbonia) redbay in the early stages of laurel wilt. USA. March 2006.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); initial attack point by Xyleborus glabratus and sapwood discoloration around entrance hole. USA. March 2005.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); initial attack point by Xyleborus glabratus and sapwood discoloration around entrance hole. USA. March 2005.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); initial attack point by Xyleborus glabratus and sapwood discoloration around entrance hole. USA. March 2005.
SymptomsRaffaelea lauricola (laurel wilt); initial attack point by Xyleborus glabratus and sapwood discoloration around entrance hole. USA. March 2005.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sapwood discoloration in a 'redbay' (Persea borbonia) in the advanced stage of laurel wilt.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); sapwood discoloration in a 'redbay' (Persea borbonia) in the advanced stage of laurel wilt.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sapwood discoloration in a 'redbay' (Persea borbonia) in the advanced stage of laurel wilt.
SymptomsRaffaelea lauricola (laurel wilt); sapwood discoloration in a 'redbay' (Persea borbonia) in the advanced stage of laurel wilt.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); discoloration in leaves and shoot of a redbay (Persea borbonia) with laurel wilt. USA. February 2005.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); discoloration in leaves and shoot of a redbay (Persea borbonia) with laurel wilt. USA. February 2005.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); discoloration in leaves and shoot of a redbay (Persea borbonia) with laurel wilt. USA. February 2005.
SymptomsRaffaelea lauricola (laurel wilt); discoloration in leaves and shoot of a redbay (Persea borbonia) with laurel wilt. USA. February 2005.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sassafras (Sassafras albidum) with laurel wilt along fence row. Georgia, USA. April 2014.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); sassafras (Sassafras albidum) with laurel wilt along fence row. Georgia, USA. April 2014.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sassafras (Sassafras albidum) with laurel wilt along fence row. Georgia, USA. April 2014.
SymptomsRaffaelea lauricola (laurel wilt); sassafras (Sassafras albidum) with laurel wilt along fence row. Georgia, USA. April 2014.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sapwood discoloration in a sassafras (Sassafras albidum) in the advance stage of laurel wilt. USA. May 2012.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); sapwood discoloration in a sassafras (Sassafras albidum) in the advance stage of laurel wilt. USA. May 2012.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); sapwood discoloration in a sassafras (Sassafras albidum) in the advance stage of laurel wilt. USA. May 2012.
SymptomsRaffaelea lauricola (laurel wilt); sapwood discoloration in a sassafras (Sassafras albidum) in the advance stage of laurel wilt. USA. May 2012.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); foliar chlorosis and wilt in pondberry (Lindera melissifolia) following infection. USA. September 2015.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); foliar chlorosis and wilt in pondberry (Lindera melissifolia) following infection. USA. September 2015.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); foliar chlorosis and wilt in pondberry (Lindera melissifolia) following infection. USA. September 2015.
SymptomsRaffaelea lauricola (laurel wilt); foliar chlorosis and wilt in pondberry (Lindera melissifolia) following infection. USA. September 2015.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); branch and shoot dieback in a camphortree (Cinnamomum camphora) infected with Raffaelea lauricola. USA. June 2007.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); branch and shoot dieback in a camphortree (Cinnamomum camphora) infected with Raffaelea lauricola. USA. June 2007.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); branch and shoot dieback in a camphortree (Cinnamomum camphora) infected with Raffaelea lauricola. USA. June 2007.
SymptomsRaffaelea lauricola (laurel wilt); branch and shoot dieback in a camphortree (Cinnamomum camphora) infected with Raffaelea lauricola. USA. June 2007.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); discoloration around Xyleborus glabratus attack points in camphortree (Cinnamomum camphora).  Raffaelea lauricola is isolated from discolored areas of the xylem. USA. April 2011.
TitleSymptoms
CaptionRaffaelea lauricola (laurel wilt); discoloration around Xyleborus glabratus attack points in camphortree (Cinnamomum camphora). Raffaelea lauricola is isolated from discolored areas of the xylem. USA. April 2011.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); discoloration around Xyleborus glabratus attack points in camphortree (Cinnamomum camphora).  Raffaelea lauricola is isolated from discolored areas of the xylem. USA. April 2011.
SymptomsRaffaelea lauricola (laurel wilt); discoloration around Xyleborus glabratus attack points in camphortree (Cinnamomum camphora). Raffaelea lauricola is isolated from discolored areas of the xylem. USA. April 2011.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); Xyleborus glabratus in xylem of redbay (Persea borbonia) and discoloration around beetle tunnel associated with Raffaelea lauricola infection. USA.
TitleVector
CaptionRaffaelea lauricola (laurel wilt); Xyleborus glabratus in xylem of redbay (Persea borbonia) and discoloration around beetle tunnel associated with Raffaelea lauricola infection. USA.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); Xyleborus glabratus in xylem of redbay (Persea borbonia) and discoloration around beetle tunnel associated with Raffaelea lauricola infection. USA.
VectorRaffaelea lauricola (laurel wilt); Xyleborus glabratus in xylem of redbay (Persea borbonia) and discoloration around beetle tunnel associated with Raffaelea lauricola infection. USA.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); gallery of Xyleborus glabratus with larva and eggs. USA. February 2012.
TitleVector
CaptionRaffaelea lauricola (laurel wilt); gallery of Xyleborus glabratus with larva and eggs. USA. February 2012.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); gallery of Xyleborus glabratus with larva and eggs. USA. February 2012.
VectorRaffaelea lauricola (laurel wilt); gallery of Xyleborus glabratus with larva and eggs. USA. February 2012.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); growth of Raffaelea lauricola in petri dish with malt extract agar. USA. March 2006.
TitleRaffaelea lauricola culture
CaptionRaffaelea lauricola (laurel wilt); growth of Raffaelea lauricola in petri dish with malt extract agar. USA. March 2006.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); growth of Raffaelea lauricola in petri dish with malt extract agar. USA. March 2006.
Raffaelea lauricola cultureRaffaelea lauricola (laurel wilt); growth of Raffaelea lauricola in petri dish with malt extract agar. USA. March 2006.©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); conidia and conidiophores of Raffaelea lauricola. USA. May 2007.
TitleConidia and conidiophores
CaptionRaffaelea lauricola (laurel wilt); conidia and conidiophores of Raffaelea lauricola. USA. May 2007.
Copyright©Stephen W. Fraedrich/United States Forest Service (USFS)
Raffaelea lauricola (laurel wilt); conidia and conidiophores of Raffaelea lauricola. USA. May 2007.
Conidia and conidiophoresRaffaelea lauricola (laurel wilt); conidia and conidiophores of Raffaelea lauricola. USA. May 2007.©Stephen W. Fraedrich/United States Forest Service (USFS)
Xyleborus glabratus (redbay ambrosia beetle); adult female. ca.2 mm in length. X. glabratus is a vector for R. lauricola.
TitleVector
CaptionXyleborus glabratus (redbay ambrosia beetle); adult female. ca.2 mm in length. X. glabratus is a vector for R. lauricola.
Copyright©U.S. Department of Agriculture (USDA)/original image by Stephen Ausmus/via flickr - CC BY 2.0
Xyleborus glabratus (redbay ambrosia beetle); adult female. ca.2 mm in length. X. glabratus is a vector for R. lauricola.
VectorXyleborus glabratus (redbay ambrosia beetle); adult female. ca.2 mm in length. X. glabratus is a vector for R. lauricola.©U.S. Department of Agriculture (USDA)/original image by Stephen Ausmus/via flickr - CC BY 2.0

Identity

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Preferred Scientific Name

  • Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva, 2008

Preferred Common Name

  • laurel wilt

Summary of Invasiveness

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Laurel wilt is responsible for the death of hundreds of millions of redbay (Persea borbonia sensu lato) trees throughout the southeastern USA, and the disease is also having significant effects on other species such as sassafras (Sassafras albidum) in natural ecosystems and avocado (Persea americana) in commercial production areas of south Florida. Laurel wilt is caused by the pathogen Raffaelea lauricola, a fungal symbiont of the redbay ambrosia beetle, Xyleborus glabratus. Thus far, the disease is confined to members of the Lauraceae that are native to the USA, or native to such places as the Caribbean, Central America and Europe and grown in the USA. The beetle and fungus are native to Asia and were likely introduced with untreated solid wood packing material at Port Wentworth, Georgia in the early 2000s. Since that time laurel wilt has spread rapidly in the coastal plains of the southeastern USA, spreading north into central North Carolina, as far west as Texas, and reaching the southernmost counties of Florida. Current models suggest that X. glabratus can tolerate temperature conditions that occur throughout much of the eastern USA, and so the disease threatens sassafras throughout much of this region. The disease poses a threat to lauraceous species indigenous to other areas of the Americas as well as Europe and Africa.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Fungi
  •         Phylum: Ascomycota
  •             Subphylum: Pezizomycotina
  •                 Class: Sordariomycetes
  •                     Subclass: Sordariomycetidae
  •                         Order: Ophiostomatales
  •                             Family: Ophiostomataceae
  •                                 Genus: Raffaelea
  •                                     Species: Raffaelea lauricola

Notes on Taxonomy and Nomenclature

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Raffaelea lauricola is the causal agent of laurel wilt, a disease that is affecting members of the Lauraceae in the southeastern USA. The genus Raffaelea is considered to be a group of asexual fungi within the Ophiostomataceae, and members of this genus are cycloheximide tolerant and symbionts of ambrosia beetles (Harrington et al., 2010).

Description

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R. lauricola has been described in detail by Harrington et al. (2008). Optimal colony growth of R. lauricola occurs at 25°C, and cream-buff and smooth colonies develop on malt extract agar that are approximately 60 mm diameter after 10 days (Harrington et al., 2008). Colonies tend to become mucilaginous in their centres and these areas are dominated by budding yeast-like conidia. Colonies that develop from spores tend to be mucilaginous initially and after several days submerged hyphae develop at the colony margins. Conidiophores are hyaline, typically aseptate, and unbranched with lengths variable, usually ranging from 13-60 µm (range 13-120 µm) and 2 µm wide (range 1-2.5 µm). The conidia are hyaline and small, typically 3.5-4.5 µm (range 3.0-8.0 µm) x 1.5-2.0 µm (range 1.0-3.5 µm) and varying from elliptical to ovoid to globose (Harrington et al., 2008).

In addition to R. lauricola, at least nine other Raffaelea species have been isolated from X. glabratus, although R. lauricola is the most common species (Harrington and Fraedrich, 2010; Harrington et al., 2010; Harrington, et al., 2011; Campbell et al., 2016). All other Raffaelea species isolated from X. glabratus are distinctly different from R. lauricola (Harrington et al., 2010) and are not pathogenic to redbay (Dreaden et al., 2017). R. lauricola has conidia and conidiophores that resemble those of R. quercivora Kubono & Shin. Ito (Kubono and Ito, 2002), with the exception that conidia of R. quercivora are broader. The two species also differ in colony growth rate and colouration (Harrington, et al., 2008). R. quercivora is the fungal symbiont of Playtypus quercivorus Murayama, an ambrosia beetle which is associated with the decline of oaks in Japan (Kubono and Ito, 2002).

Distribution

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R. lauricola is presently known to occur in South East Asia and the southeastern region of the USA. R. lauricola is a fungal symbiont of the redbay ambrosia beetle, Xyleborus glabratus (Fraedrich et al., 2008; Harrington et al., 2008; Harrington and Fraedrich, 2010), and the beetle is native to Asia where it has been documented in Japan, Taiwan, China, India, and Myanmar (Wood and Bright, 1992; Hulcr and Lou, 2013). Raffaelea lauricola has been documented with X. glabratus beetles from Taiwan and Japan as well as the USA (Harrington et al., 2011) and it is assumed that the fungus is associated with X. glabratus elsewhere in Asia. R. lauricola is not known to cause a vascular, systemic wilt or other disease in native plants in Asia. In 2014, R. lauricola was isolated from avocado (P. americana) trees exhibiting symptoms of laurel wilt in Myanmar (Ploetz et al., 2016); avocado is an introduced species that is cultivated in Myanmar.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

ChinaPresentNative Not invasive Hulcr and Lou, 2013Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
-FujianPresentNative Not invasive Hulcr and Lou, 2013Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
-HunanPresentNative Not invasive Hulcr and Lou, 2013Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
-SichuanPresentNative Not invasive Hulcr and Lou, 2013Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
IndiaPresentNative Not invasive Wood and Bright, 1992Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
-AssamPresentNative Not invasive Wood and Bright, 1992Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
-West BengalPresentNative Not invasive Wood and Bright, 1992Only Xyleborus glabratus noted here, no attempt to look for Raffaelea lauricola
JapanPresentNative Not invasive Harrington et al., 2011Raffaelea lauricola found in association with Xyleborus glabratus
-KyushuPresentNative Not invasive Harrington et al., 2011Raffaelea lauricola found in association with Xyleborus glabratus
MyanmarPresentNative Not invasive Wood and Bright, 1992; Ploetz et al., 2016
TaiwanPresentNative Not invasive Harrington et al., 2011Lienhuachih, Fushan, Hsinshien; Raffaelea lauricola found in association with Xyleborus glabratus

North America

USAWidespreadIntroduced Invasive Fraedrich et al., 2008Southeastern USA
-AlabamaWidespreadIntroduced Invasive Bates et al., 2013Southern and central portions of the state
-ArkansasWidespreadIntroduced Invasive Olatinwo et al., 2016Southern portion of the state
-FloridaWidespreadIntroduced Invasive Fraedrich et al., 2008Throughout the state
-GeorgiaWidespreadIntroduced Invasive Fraedrich et al., 2008Coastal Plains
-LouisianaWidespreadIntroduced Invasive Fraedrich et al., 2015aNorthern and western portions of the state
-MississippiLocalisedIntroduced Invasive Riggins et al., 2010Gulf Coast
-North CarolinaWidespreadIntroduced Invasive Forest Health, 2018Coastal Plains
-South CarolinaWidespreadIntroduced Invasive Fraedrich et al., 2008Coastal Plains
-TexasWidespreadIntroduced Invasive Menard et al., 2015Eastern portions of the state

History of Introduction and Spread

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X. glabratus was first intercepted in 2002 when several beetles were caught in ethanol-baited traps at Port Wentworth, Georgia, as part of an ‘early detection and rapid response’ program that was funded by the US federal government in cooperation with state forestry agencies (Rabaglia et al., 2006; Rabaglia et al., 2008). It is believed that X. glabratus was probably already well established around Port Wentworth at the time of the first intercept because X. glabratus does not respond to ethanol lures (Johnson et al., 2014; Kendra et al., 2014), and dead redbay trees had been noted in the area before 2002 (Hanula and Sullivan, 2008). There had been no precedent at this time for a fungal symbiont of an ambrosia beetle to cause a lethal, systemic vascular wilt, and thus, any mortality in redbay that occurred around Port Wentworth in the early 2000s could have gone unnoticed for years until the problem was finally investigated on Hilton Head Island, South Carolina in 2004.

It is assumed that R. lauricola was introduced with X. glabratus. In November 2004, R. lauricola was first isolated from dying redbay (Persea borbonia) trees on Hilton Head Island, South Carolina (Fraedrich et al., 2008; Harrington et al., 2008), located approximately 25 miles from Port Wentworth. X. glabratus was also recovered from branches and stems of the dead and dying redbay at this time. R. lauricola and X. glabratus were subsequently associated with mortality of redbay trees at numerous additional locations in the coastal plain forests of South Carolina, Georgia and Florida during 2005 (Fraedrich et al., 2008). Since this time, the beetle and fungus have continued to spread rapidly on redbay and sassafras throughout the Atlantic and Gulf coastal plains of the southeastern USA, and has spread much faster than original models had predicted (Koch and Smith, 2008). By 2011, laurel wilt had spread southward into southern Florida, where the disease threatened avocado production areas (Ploetz et al., 2011a). The disease also moved rapidly westward and was found in Mississippi in 2009 (Riggins et al., 2010), Alabama in 2011 (Bates et al., 2013; Formby et al., 2012), Louisiana in 2014 (Fraedrich et al., 2015a), and Arkansas and Texas in 2015 (Menard et al., 2016; Olatinwo et al., 2016). Movement of laurel wilt southward and westward has often been characterized by large ‘jumps’, with development of satellite disease spots often located many hundreds of kilometres from areas where the disease was previously known to occur (Riggins et al., 2010; Bates et al., 2013; Fraedrich et al., 2015a; Menard et al., 2016). Northward movement of laurel wilt has been comparatively slow. Although laurel wilt has been present in North Carolina since 2011, the state’s northern most coastal counties are presently not affected by the disease, despite the presence of abundant redbay in these areas. The United States Forest Service maintains a website that documents the spread of laurel wilt by state, county and year (https://www.fs.usda.gov/main/r8/forest-grasslandhealth).

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
USA Asia Early 2000s Timber trade (pathway cause) Yes No Harrington et al. (2011) Likely cause of introduction, but not proven

Risk of Introduction

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The spread of R. lauricola and X. glabratus into new areas over long and short distances appears to be largely dependent on the movement of infested wood. The beetle and fungus were most likely introduced to the USA with solid wood packaging material from Asia (Rabaglia et al., 2008, Harrington et al., 2011). Among the possible factors contributing to the subsequent spread of the beetle and fungus within the USA include the movement of infested timber to wood product mills, movement of firewood by the general public, and hobbyists such as wood workers who may collect and transport infested wood over long distances (Fraedrich et al., 2015a; Hughes et al., 2015; Ploetz et al., 2017a).

X. glabratus appears to be primarily associated with species in the family Lauraceae in South East Asia, although there have been scattered reports that X. glabratus is also associated with some species in the families Dipterocarpaceae, Fagaceae, Theacae, Pinaceae and Fabaceae (Wood and Bright, 1992; Rabaglia et al., 2006; Hulcr and Lou, 2013). In the USA, X. glabratus has only been associated with members of the Lauraceae (Hughes et al., 2015), where its fungal symbiont, R. lauricola, is the cause of wilt in species that are native to the southeastern USA as well as introduced species such as avocado and bay laurel (Laurus nobilus).

Members of the Lauraceae occur in tropical regions throughout the world with a few species occurring in temperate regions (Rohwer, 1993). Areas of the world that are potentially at greatest risk if X. glabratus and R. lauricola are introduced include:

  1. South America, Central America and the islands of the Caribbean, where there is great species diversity in the Lauraceae and where numerous tree species of this family occur in forest ecosystems (Rohwer, 1993). This region is thought to have more than 750 lauraceous species in 26 genera (van der Werff, 1991). In the cloud forests of Mexico, there are at least 68 lauraceous tree species that occur in 8 genera including many species of Persea and Litsea (González-Espinosa et al., 2011). In addition, Mexico is the world’s largest producer of avocados, and other countries such as the Dominican Republic, Peru, Columbia and Brazil are all major avocado producers.
  2. The west coast of the USA, where California laurel (Umbellularia californica) is native to the Sierra Nevadas and the coastal forests of California and Oregon. Avocado is also grown commercially in southern California.
  3. The Macaronesian Islands (i.e. Azores, Canary Islands, Madeira) where laurel forests are dominated by species of Laurus, Persea, Apollonias and Ocotea. The laurel forests of Madeira have been designated as a UNESCO World Heritage Site and are considered a relict, providing an excellent example of a forest type that was once common throughout Southern Europe (https://whc.unesco.org/en/list/934).
  4. The Mediterranean areas of Europe, Asia and Africa where bay laurel is native in some forest types.

In addition, lauraceous species occur in Australia and other parts of Africa, although little is known at this time about the susceptibility of species that occur in these regions of the world.

Habitat

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Little is known about the biology and ecology of X. glabratus in Asia but the beetle is known to prefer species in the Lauraceae. Thus, R. lauricola and X. glabratus are likely to occur in Asian forests where lauraceous species are present.

In the southeastern USA, laurel wilt is now widespread and affecting redbay and sassafras in forest types where these susceptible hosts are present. Landscape trees in parks and in residential areas are also affected. Avocado, grown commercially in orchards in south Florida and in residential areas as an ornamental, are also being killed by the disease.

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial
Terrestrial – ManagedCultivated / agricultural land Secondary/tolerated habitat Harmful (pest or invasive)
Urban / peri-urban areas Secondary/tolerated habitat Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural forests Principal habitat Harmful (pest or invasive)
Natural forests Principal habitat Natural

Hosts/Species Affected

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Many members of the Lauraceae that are native to the southeastern USA appear to be highly susceptible to laurel wilt, although some have not been greatly impacted by the wilt for various reasons. A couple of species in the Lauraceae that are native to Florida appear to be somewhat resistant to the disease. Species indigenous to South East Asia appear to be mostly resistant to laurel wilt. A more complete assessment of what is known about the susceptibility of the individual species follows.

Persea borbonia (redbay) and P. palustris (swampbay) are very similar taxa with differentiating characteristics that are vague and not always reliable (Coker and Toten, 1945). Some have regarded the two taxa as the same species or varieties of a species (Radford et al., 1968; Little, 1979) while others consider them to be separate species (Shearman et al., 2018; Weakley 2015). Regardless, redbay and swampbay, historically treated by many as one species (Radford et al., 1968; Brendemuehl, 1990), appear to be equally susceptible to laurel wilt (Fraedrich et al., 2008), and are difficult to accurately distinguish under field conditions. Thus, redbay is treated in this database as a single species (i.e. P. borbonia sensu lato). Redbays are evergreen trees that occur in the coastal plain forests of the southeastern USA, and are a minor use hardwood and ecologically important in the forest ecosystems where they occur (Brendemuehl, 1990). Redbay has been affected by laurel wilt throughout much of its range with losses that are estimated into the hundreds of millions of trees (Hughes et al., 2017). The disease preferentially affects larger diameter trees (Fraedrich et al., 2008; Mayfield and Brownie, 2013) and throughout its range there is still high survivorship among smaller diameter redbay trees as well as sapling and seedlings (Cameron et al., 2015). The reason for this phenomenon is thought to be due to the preference of X. glabratus to attack larger diameter trees (Mayfield and Brownie, 2013) and not due to resistance in the smaller diameter plants (Fraedrich et al., 2008). Thus, although redbay trees have been devastated by laurel wilt in the southeastern USA, it does not appear that the species is in imminent danger of extinction. The long term survival of redbay in the southeastern USA will depend on the ability of X. glabratus to find and reproduce in smaller diameter redbays or other suitable hosts.

Sassafras albidum (sassafras) is a deciduous tree species that occurs in various forest types over much of the eastern half of the USA (Griggs, 1990; Randolph, 2017) and is a minor use hardwood (Harding et al., 1997; Cassen, 2007). Pathogenicity tests confirmed that sassafras is highly susceptible to laurel wilt (Fraedrich et al., 2008) and the disease has affected sassafras across the southeastern USA (Bates et al., 2013; Fraedrich et al., 2015a, Olatinwo et al., 2016). Recent studies indicate that X. glabratus can survive the low winter temperatures throughout much of the range of sassafras (Formby et al., 2018), however at this time, the northern most location of the disease is central North Carolina (Mayfield et al., 2019).

Persea americana (avocado) is a tropical evergreen tree that is native to Central America and the Caribbean. The species is cultivated for production of avocados in Florida and California in the USA, and is also grown in Mexico and many other countries. Three distinct races of avocado are recognized that include the Mexican, Guatemalan and West Indian. The West Indian and West Indian-Guatemalan hybrids are primarily cultivated for commercial production in Florida (Mayfield et al., 2008a). Some avocado cultivars are more susceptible to laurel wilt than others, and West Indian cultivars such as ‘Simmonds’ appear to be highly susceptible (Mayfield et al., 2008a; Ploetz et al., 2012a). The ‘Simmonds’ cultivar comprises approximately 35% of the avocado production in Florida (Ploetz et al., 2011b). The West Indian-Guatemalan hybrids are generally susceptible but less so than the West Indian cultivars, and Guatemalan x Mexican hybrids such as the ‘Hass’ cultivar appear to be among the most resistant (Mayfield et al., 2008a; Ploetz et al., 2012a). The ‘Hass’ cultivar accounts for 95% of all production in California (Ploetz et al., 2017a).

Persea humilus (silk bay) is another species for which the taxonomy is confused. Some would regard silk bay as a species (Nelson, 1994), while others regard this taxon as a variety of redbay (Persea borbonia var. humilus) (Wunderlin, 1998). Silk bay is a small evergreen tree that is native to the scrub forests of central Florida. Laurel wilt is currently affecting silk bay in forests, and its susceptibility to the disease has been confirmed through pathogenicity tests (Hughes et al., 2012).

Lindera melissifolia (pondberry) is a small, deciduous, clonal shrub that is extremely rare and listed as an endangered species in the USA. Pathogenicity tests have determined that pondberry is highly susceptible to laurel wilt, but the disease has only been observed once in this species under natural conditions (Fraedrich et al., 2011). Because of its small stem diameter, pondberry is not readily attacked by X. glabratus. However, because of the clonal nature of this species, when infections occur, the disease can spread rapidly through rhizomes and kill multiple ramets within a population (Best and Fraedrich, 2018).

Lindera benzoin (spicebush) is a common small, deciduous shrub species that is found in the southeastern USA, and in pathogenicity tests it proved to be highly susceptible to laurel wilt (Fraedrich et al., 2008). The disease has been documented only once naturally in spicebush (Fraedrich et al., 2016), and because of the small diameter of spicebush, it is not readily attacked by X. glabratus. Therefore, the disease does not appear to be a major threat to this species.

Litsea aestivalis (pondspice) is a relatively large (0.5-3 m tall) deciduous, multi-branched shrub that occurs in the southeastern USA. The species is rare and is listed as threatened. Pondspice is highly susceptible to laurel wilt (Fraedrich et al., 2011), but due to the small size of this species, it is not readily attacked by X. glabratus. Laurel wilt has been observed in pondspice in Georgia and South Carolina (Fraedrich et al., 2011) and Florida (Hughes et al., 2011).

Licaria trianda (pepperleaf sweetwood) is a rare, evergreen tree native to the lower, southeastern portion of Florida. The species is considered to be endangered. A pathogenicity study determined that pepperleaf sweetwood was susceptible to disease caused by R. lauricola. Leaves of infected seedlings developed chlorosis and abscised, and a brown discolouration was noted in the xylem of stems. However, seedlings did not die from the disease (Ploetz and Konkol, 2013).

Ocotea coriacea (lancewood) is a small, evergreen tree that is found at scattered locations in central to south Florida and elsewhere in Central America and the Caribbean. Saplings inoculated with R. lauricola develop discolouration in the xylem and occasionally dieback of the branches but saplings do not die (S Fraedrich, US Forest Service, Georgia, USA, unpublished data).

Umbellularia californica (California laurel) is a large evergreen tree species native to southwestern Oregon, and the Coastal Ranges and Sierra Nevada of California. In laboratory pathogenicity tests, R. lauricola-inoculated plants developed sapwood discolouration and branch dieback but plants did not die from wilt (Fraedrich, 2008). A subsequent study also found that California laurel was an excellent brood host for X. glabratus (Mayfield et al., 2013).

Laurus nobilus (bay laurel) is an evergreen tree or large shrub species that is native to the Mediterranean regions of Europe, Asia and Africa. The taxonomy of Laurus nobilis and a similar species, L. azorica, which is found in Madeira and the Canary Islands, is confused and in need of review (Arroyo-García et al., 2001). Laurus nobilus was introduced into the USA, where it has been cultivated as a culinary herb and valued as a landscape ornamental species. Laurel wilt has been observed in a landscape plant in Florida and susceptibility of the bay laurel to the disease was subsequently confirmed in pathogenicity tests (Hughes et al., 2014).

Persea indica (viñatigo) is an evergreen tree native to the maritime forests of the Canary Islands, Madeira and the Azores. Viñatigo has been used as an ornamental in Florida and California in the USA (Schuch et al., 1992), and the species has been shown to be susceptible to laurel wilt in field and laboratory experiments (Hughes et al., 2013).

Cinnamomum camphora (camphortree) is indigenous to China, Japan, Taiwan and other countries in eastern Asia. The species was introduced into the southeastern USA in the 1800s and was used for camphor production, but has escaped cultivation and is now naturalized in some forest types (Langeland et al., 2008). Camphortree appears to be highly resistant to laurel wilt. Reports of laurel wilt in camphortree are not known in Asia, and in the USA the disease rarely affects camphortree in areas where redbay populations have been decimated by laurel wilt. Dieback in camphortrees is occasionally observed in trees where laurel wilt is prevalent on redbay, and R. lauricola has been recovered from such trees (Smith et al., 2009; Fraedrich et al., 2015b). Single point inoculations of camphortree saplings with R. lauricola do not produce symptoms of laurel wilt or dieback under controlled conditions; however, multiple inoculations with R. lauricola have resulted in top dieback and mortality in saplings (Fraedrich et al., 2015b).

In addition, a study of the susceptibility of lauraceous species native to South East Asia, indicated that Cinnamomum osmophloeum, C. jensenianum, Machilus zuihoensis and M. thunbergii were also much more resistant to laurel wilt than species native to North America (Shih et al., 2018).

Growth Stages

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Symptoms

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R. lauricola moves rapidly in the xylem of trees (Fraedrich et al., 2015a) and disease symptoms are often observed in portions of redbay trees within two to four weeks after infection. The disease then spreads throughout the entire crown, and redbay trees typically wilt completely within 4 to 12 weeks following inoculation (Fraedrich et al., 2008; Mayfield et al., 2008b). Leaves of infected trees initially droop from loss of turgor and then turn a reddish-brown colour as they die. Some older leaves may initially become chlorotic as they are dying. Leaves on redbay and some other evergreen hosts do not abscise after dying and can be retained on branches for a year or more after the tree has died. A dark black discolouration is observed in the stem and branch sapwood of infected plants. The discolouration is initially observed in the outermost sapwood as localized streaks in the early stages of wilt, but over time the discolouration occurs more extensively throughout the cross-sectional area of the xylem tissue. Symptom development is similar in sassafras and other deciduous hosts except leaves are likely to drop as they die or soon after (Fraedrich et al., 2008; Fraedrich et al., 2015a). Sassafras leaves take on a reddish discolouration (Fraedrich et al., 2008) and pondberry leaves become very chlorotic and turn a bright yellow as they die (Best and Fraedrich, 2018).

The rate of development of the disease and subsequent symptoms in redbay plants depends greatly on their size and environmental factors, such as temperature and moisture conditions. The disease appears to progress relatively slow in trees infected late in the growing season, and trees with partial crown wilt on only a few branches can be found during the winter months. The disease progresses much more rapidly, and trees die quickly, during the spring and summer months when trees are actively transpiring and growing.

Symptom development in avocado is somewhat similar to that of redbay. The first symptom is the wilting of terminal leaves and the development of brown-to-black discolouration as they die (Ploetz et al., 2017a). Unlike redbay, leaves of avocado tend to abscise within 2 to 9 months following symptom development (Ploetz et al., 2017a). Apparently symptoms can be localized in avocado trees with the disease affecting some branches in portions of trees, and epicormic sprouting beneath the affected portions of trees can subsequently lead to the production of healthy branches (Ploetz et al., 2017a). The sapwood of infected trees develops a brown to black discolouration (Mayfield et al., 2008c).

List of Symptoms/Signs

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SignLife StagesType
Growing point / dieback
Growing point / wilt
Leaves / wilting
Leaves / yellowed or dead
Stems / internal discoloration
Stems / visible frass
Stems / wilt
Whole plant / discoloration
Whole plant / frass visible
Whole plant / plant dead; dieback
Whole plant / wilt

Biology and Ecology

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Genetics

The genetics of R. lauricola have not been extensively studies and at this time little is known. The fungus is only known to reproduce asexually from conidia produced on conidiophores and from yeast-like budding of conidia. Although a teleomorphic stage has not been found in R. lauricola, a high degree of genetic diversity has been observed in populations of R. lauricola from Taiwan and Japan, and isolates from these countries can be separated into MAT-1 and MAT-2 mating types, similar to that which has been observed in other species in the Ophiostomatales (Wuest et al., 2017). Thus, sexual reproduction may be occurring in R. lauricola populations in Asia, although cryptically as has been documented for other fungal species (Kuck and Poggeler, 2009). Very little variation is observed in the genetic diversity of the R. lauricola population that is present in the USA, supporting the hypothesis that the current laurel wilt epidemic is due to a single introduction (Wuest, et al., 2017).

Climate

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ClimateStatusDescriptionRemark
Aw - Tropical wet and dry savanna climate Preferred < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
Cf - Warm temperate climate, wet all year Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year
Df - Continental climate, wet all year Tolerated Continental climate, wet all year (Warm average temp. > 10°C, coldest month < 0°C, wet all year) Predicted, see Formby et al. (2018)

Means of Movement and Dispersal

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Natural Dispersal

The spread of laurel wilt through roots of susceptible hosts has not been well studied, but this means of transmission appears to vary greatly among the various hosts. Movement of R. lauricola through roots among redbay trees is a possibility, but patterns of mortality in the forests suggest this may not be an important mode of transmission for this species (Cameron et al., 2015). Transmission of R. lauricola through roots of sassafras is very likely. Sassafras reproduces vegetatively through root suckers and patterns of disease development in forests suggest the disease is spreading through roots (Cameron et al., 2015). A study of laurel wilt transmission in pondberry, a clonal species, found that R. lauricola could spread rapidly through rhizomes, killing as many as 59 ramets at distances as great as four metres from the plant initially infected by the pathogen (Best and Fraedrich, 2018). In avocado orchards, the spread of R. lauricola through root grafts between avocado trees is thought to be an important means of transmission because of the rapid rate at which disease foci develop around recently infected trees (Ploetz et al., 2017a).

Vector Transmission

The primary means of transmission of R. lauricola is by its symbiotic partner, X. glabratus, an ambrosia beetle native to Asia. The fungus is carried by the beetle in mycangia located in the head of the beetle behind the mandibles (Fraedrich et al., 2008). In the USA, X. glabratus adult beetles make their initial attacks on the stems of the healthy redbay trees, at which time the fungal pathogen is transmitted and trees become infected (Fraedrich et al., 2008). Single beetle attacks are sufficient to inoculate redbay plants and cause wilt, and single inoculation points with R. lauricola are sufficient to cause wilt and mortality in redbay trees (Fraedrich et al., 2008; Mayfield et al., 2008b). The initial attacks by the beetle on healthy trees often appear to be aborted (Fraedrich et al., 2008; Fraedrich et al., 2015a; Hughes et al., 2015). It is not until after redbay trees have wilted that they are mass attacked by X. glabratus and used for brood production.

X. glabratus uses visual and olfactory cues to select hosts. The beetle prefers to attack larger diameter hosts and visual cues have been shown to play a role in the selection process (Mayfield and Brownie, 2013). Unlike many ambrosia beetles which are attracted to ethanol, a volatile given off by stressed and dying trees, X. glabratus is attracted to host volatiles that are produced by redbay and other members of the Lauraceae (Hanula and Sullivan, 2008; Hanula et al., 2008). Among the many volatiles produced by redbay, the beetle appears to be particularly attracted to the sequiterpene, α-copaene.

Populations of X. glabratus increase rapidly in areas where abundant large diameter redbays are dying from laurel wilt, but after a couple of years, beetle populations decrease dramatically (Maner et al., 2014). Over time, the xylem of hosts becomes invaded by secondary organisms and the wood becomes increasingly unsuitable for reproduction by X. glabratus.

Avocado does not appear to be a good reproductive host for X. glabratus, although the beetle is somewhat attracted to avocado and will bore into avocado wood (Mayfield et al. 2008a; Carrillo et al., 2012; Mayfield and Hanula 2012; Pena et al., 2012; Brar et al., 2013). In avocado orchards, X. glabratus has been recovered from only a low percentage of avocado trees with wilt-like symptoms (Carrillo et al., 2012) and the beetle is rarely trapped in avocado orchards (Ploetz et al., 2017a; Menocal et al., 2018). R. lauricola is frequently isolated from other ambrosia beetles (e.g. X. volvulus and X. ferrugineus) that colonize wilted avocado trees, and it is suspected that some of these beetles may be responsible for transmission of the pathogen to healthy avocado trees (Carrillo et al., 2014; Ploetz et al., 2017b).

Seedborne Aspects

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R. lauricola is not known to be transmitted in fruit or seeds. Ploetz et al. (2012b) did not find R. lauricola in fruit pulp and seeds of avocados from infected trees, although the fungus could be isolated from the pedicels/peduncles and hila of fruit. Attempted isolations from pondberry and swampbay plants infected with R. lauricola, also failed to find the pathogen in fruits and seeds (GS Best and SW Fraedrich, US Forest Service, Georgia, USA, unpublished data).

Wood Packaging

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Wood Packaging liable to carry the pest in trade/transportTimber typeUsed as packing
Solid wood packing material with bark Members of the Lauraceae, possibly species in other families Yes
Solid wood packing material without bark Members of the Lauraceae, possibly species in other families Yes

Vectors and Intermediate Hosts

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VectorSourceReferenceGroupDistribution
Xyleborus glabratusWood and Bright, 1992; Rabaglia et al., 2008; Hulcr and Lou, 2013. InsectTaiwan

Economic Impact

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Sassafras and redbay tend to be minor use hardwoods and economic impacts from the loss of these species are expected to be minor, with potentially localized impacts in some areas.

The wood of redbay is strong, hard, and has a reddish colour, and has been used for furniture and interior finishing, and is also used by wood workers for turning bowls and similar items, but in general, the wood is not commercially available. Redbays are regarded as excellent landscape trees with many benefits (Gilman and Watson, 1994a). In contrast to redbay, sassafras wood appears to be more readily available, and is used for furniture, millwork and paneling (Harding et al., 1997; Cassen, 2007). Sassafras is also an excellent landscape tree and valued as an ornamental (Gilman and Watson, 1994b).

The production of avocado in the USA has not been significantly affected by laurel wilt at this time, although the threat remains. Florida produces approximately 14% of the total USA avocado production (Ploetz et al., 2011b), and it is estimated that approximately 2% of the avocado trees in commercial production areas of Florida have been lost to laurel wilt (Ploetz et al., 2017a).

Environmental Impact

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Redbay continues to survive in forests of the southeastern USA as small trees and saplings, and there is significant sprouting from stumps of trees that have died from laurel wilt (Fraedrich et al., 2008; Shields et al., 2011; Cameron et al., 2015). Nonetheless, Evans et al. (2014) expressed concerns that redbay could become ‘ecologically extinct’ and will no longer function as a significant component in the ecosystems where they occur. The loss of redbay is changing forest structure that could have subsequent effects on herbaceous composition and productivity, reductions in sub-canopy stem density leading to less nesting habitat for birds, and effects on nutrient cycling (Evans et al., 2014). Other species such as sweetbay (Magnolia virginiana) and loblolly bay (Gordonia lasianthus), that occupy a similar niche to redbay in the Georgia Coastal Plains, and are often co-dominant in stands were redbay occurs, may be increasing in dominance following redbay mortality (Spiegel and Leege, 2013). In the Florida Everglades there are concerns that the canopy disturbance following mortality of swampbay (Persea palustris) will lead to increased colonization of invasive species such as the old world climbing fern (Lygodium microphyllum) and the Brazilian pepper (Schinus terebinthifolius) (Rodgers et al., 2014). Swampbay is a major canopy component of the tree islands in the Everglades, and there are also concerns that the sudden loss of this species could reduce the rates of peat accretion and cause instability of the islands (Rodgers et al., 2014).

Impact: Biodiversity

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The long-term effects that the loss of redbay trees will have on forest ecosystems are largely unknown. The fruit of redbay is eaten by songbirds and game birds such as turkey and quail, and bear and deer browse on redbay foliage and fruit (Goodrum, 1977; Coder, 2012; Chupp and Battaglia, 2016). However, these species are generalists and it is not clear how the loss of redbay would affect various animal populations. One species that may be affected by the loss of redbay is the Palamedes swallowtail butterfly (Papilio palamedes). Redbay serves as the primary larval host of this butterfly, and recent findings suggest that the abundance of this butterfly species is lower in forests where laurel wilt has affected redbay populations (Chupp and Battaglia, 2014; Riggins et al., 2018). Other insect species have been also identified as ‘Lauraceae specialists’ and the loss of redbay, or other lauraceous species, could also impact these arthropods (Riggins et al., 2018).

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Lindera melissifoliaNo DetailsUSAPathogenicFraedrich et al., 2011 US endangered species list
Litsea aestivalisNo DetailsUSAPathogenicFraedrich et al., 2011
Licaria triandraNo DetailsFloridaPathogenicPloetz and Konkol, 2013

Social Impact

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Leaves, wood and bark of many species in the Lauraceae have been used worldwide for spices, medicinal purposes and for making perfumes (Bailey, 1949; Kostermans, 1957). These uses may be impacted by laurel wilt. In southeastern USA, Native American tribes have used redbay to treat various illnesses and the species is still used in tribal ceremonies (Hughes et al., 2015). The leaves of redbay are highly aromatic and are used to season traditional dishes of the southern USA, such as gumbo (Goodrum, 1977; Coder, 2012). Sassafras has also been used extensively by Native American tribes to treat various illnesses including rheumatism, scarlet fever, coughs and colds (Immel, 2003). The oil from the bark and roots of sassafras has been used in making soaps and perfumes, and the plant was introduced into Europe for use in pharmaceuticals (Harrar and Harrar, 1962). The leaves of sassafras are dried and ground to make ‘file’, a spice that is used to season and thicken Cajun dishes (Immel, 2003). Sassafras tea and beer is brewed from roots and twigs (Harrar and Harrar, 1962), but safrole, an aromatic oil in the root bark of sassafras, is believed to be a carcinogen and has been banned for use in foods in the USA (Segelman et al., 1976).

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Highly mobile locally
  • Reproduces asexually
Impact outcomes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Host damage
  • Increases vulnerability to invasions
  • Modification of successional patterns
  • Negatively impacts agriculture
  • Negatively impacts cultural/traditional practices
  • Negatively impacts forestry
  • Negatively impacts livelihoods
  • Reduced native biodiversity
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species
Impact mechanisms
  • Pathogenic
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field
  • Difficult/costly to control

Diagnosis

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Malt extract agar amended with cycloheximide and streptomycin sulfate (CSMA) is the best media for isolation of R. lauricola from wood samples of plants suspected of having laurel wilt (Harrington, 1981; Harrington, 1992; Fraedrich et al., 2008). Samples of discoloured sapwood are surface sterilized, small chips of the wood are placed on the selective media, and plates are incubated at 25°C. R. lauricola typically grows from chips in 3 to 10 days, possibly varying based on the level of colonization of samples by the pathogen and the host affected. Identification of R. lauricola is based on its unique mucoid colony growth on agar media, conidiophores and the size and shape of its buddings conidia (Harrington et al., 2008). Because R. lauricola is a recently discovered species and our understanding of species in the genus Raffaelea and similar taxa is still developing, additional confirmation of R. lauricola is often obtained using molecular identification techniques. These techniques, as well as pathogenicity tests, are often used when confirming the presence of R. lauricola and laurel wilt in a new area or on a new host (Bates et al., 2013; Fraedrich et al., 2015a; Fraedrich et al., 2016). The use of large subunit (LSU, 28S) RNA primers, PCR amplification and sequencing has been used extensively to confirm R. lauricola cultures isolated from samples (Harrington et al., 2010; Harrington et al., 2011; Jeyaprakash et al., 2014). Another procedure that uses primers developed from microsatellite loci has been also developed and is employed by some laboratories (Dreaden et al., 2014).

Detection and Inspection

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The detection of laurel wilt in redbay and sassafras is usually straightforward with wilted and dead foliage occurring in some branches initially and eventually over the entire crowns of trees (Fraedrich et al., 2008). A black discolouration is observed in the sapwood of stems and branches. Initially, the discolouration is primarily evident in the outermost sapwood but as the disease progresses the discolouration will be observed through much of the cross-sectional area of the sapwood. Isolation of the pathogen from infected tissues on agar media is necessary to confirm the disease diagnosis. Frass tubes are typically observed on stems and branches of redbay and other species being attacked by X. glabratus, and frequently these are numerous after trees have wilted.

Similarities to Other Species/Conditions

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Damage to redbay and pondberry plants caused by the black twig borer, Xylosandrus compactus, can be confused with the early stages of laurel wilt. The beetle, another invasive ambrosia beetle from Asia, attacks healthy twigs and small branches of plants, causing the wilt of foliage on terminal shoots and subsequent dieback (Dixon and Woodruff, 1983). The beetle does not carry a highly pathogenic fungus and the damage affects single twigs or branches, although multiple attacks are usually observed in the crown of redbay.

Sassafras is reported to be susceptible to verticillium wilt (Sinclair and Lyon, 2005), although this does not appear to be a widespread problem. The discolouration of the sapwood is likely to be similar to that which develops in sassafras trees affected by laurel wilt, and laboratory culture would be necessary to confirm diagnosis.

Avocado is also susceptible to verticillium wilt as well as phytophthora root rot, and symptoms in avocado can resemble those of laurel wilt. In addition, abiotic factors such as lightning strikes and freeze damage could also cause wilt-like symptoms and need to be considered when diagnosing damage and mortality in avocado (Ploetz et al., 2017a).

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

The development, implementation and enforcement of sanitary and phytosanitary measures with respect to wood movement are critical to preventing the further spread of laurel wilt worldwide. The use of untreated wood packing material in international trade is mostly likely responsible for the introduction of X. glabratus and R. lauricola into the USA from Asia (Rabaglia et al., 2008), and wood movement by industry and the general public are most likely responsible for the subsequent movement of the beetle and pathogen over long distances within the USA (Fraedrich et al., 2008; Fraedrich et al., 2015a). Thus, preventing the movement of untreated wood from areas where X. glabratus and R. lauricola are present, to areas where there are potentially susceptible host species, is imperative. Currently, the movement of wood packaging material is regulated internationally under the International Standards for Phytosanitary Measures (ISPM 15) (FAO, 2017). There are currently no quarantines that restrict the movement of wood from redbay, sassafras or other lauraceous species among states in the USA; however, the state of Florida has placed restrictions on the movement of firewood within the state, and no unprocessed wood material is permitted to be moved into Miami-Dade County, where avocados are produced commercially (Ploetz et al., 2017a).

Early Warning Systems

The development of early warning systems could be instituted at port facilities and other locations where X. glabratus and R. lauricola are likely to gain entry to a country or a new region. Traps baited with lures containing high levels of α-copaene are most effective for trapping X. glabratus (Hanula et al. 2013; Kendra et al. 2016a; Kendra et al. 2016b) and could provide plant health inspectors with the means for early detection of the beetle and subsequent response before it becomes well established. A general knowledge of the species of plants that are potentially susceptible to laurel wilt and disease symptoms are needed by plant health officials and the general public in order to increase the probability that laurel wilt is detected early at new locations and does not go unnoticed for an extended time period.

Control

Once X. glabratus is established in natural ecosystems that have susceptible host species, laurel wilt is difficult, if not impossible, to control. So far, attempts to control laurel wilt in the USA, through sanitation cuts, have not been successful. A sanitation cut and destruction of the felled redbay trees with laurel wilt on a barrier island in Georgia failed to stop the laurel wilt epidemic, and a rigorous program that promptly removed diseased and infested trees in an island community near Charleston, South Carolina did little to stop disease progression. Sanitation procedures such as the timely removal of trees with laurel wilt, and chipping, burning and burying infested material, will probably help to reduce the rate of spread of the disease but it is doubtful that such practices will eradicate the beetle or pathogen once they are well established. Chipping wood from wilted redbay trees has been found to greatly reduce beetle populations and decrease R. lauricola survival in chips, but the practice will not eradicate the beetle (Spence et al., 2013). The use of contact insecticides on chips could help to further reduce beetle populations (Carrillo et al., 2013; Ploetz et al., 2017a). Previous efforts to control the disease through sanitation most likely failed because of the inability to eliminate all beetles during sanitation cuts and because of the movement of beetles from untreated areas into those being treated. Studies have found that X. glabratus can disperse widely, and populations of the beetle remain uniformly high for more than 300 metres from areas with infested trees that have died from laurel wilt (Hanula et al., 2016). Unfortunately, the survival of even one female X. glabratus beetle in areas where the disease is trying to be controlled is sufficient to reestablish a beetle population and continue the epidemic (Haack and Rabaglia, 2013).

Tree injections with propiconazole have been shown to protect redbay trees from laurel wilt (Mayfield et al., 2008b), and this practice could be used to protect high value landscape trees; however, tree injections are expensive and would need to be repeated every one or two years. Recent research suggests that some redbays that survive laurel wilt epidemics may have resistance to the pathogen, and propagation of resistant hosts may be viable as part of a long-term management or restoration strategy (Hughes et al., 2015). At this time, it is unclear if X. glabratus will continue to survive in smaller diameter redbay plants that still occur in areas that have been affected by the disease.

The management of laurel wilt in avocado orchards is dependent on the early detection of the disease, because the movement of R. lauricola through root systems of infected trees to adjacent trees can occur rapidly. Currently, the detection of infected trees is dependent on observing symptom development in trees (Ploetz et al., 2017c) but the use of canines trained to detect the scent of avocado trees infected with R. lauricola before they become symptomatic has shown promise (Mendel et al., 2018). Once trees with laurel wilt are detected, their prompt removal is essential. The main stems and branches of trees should be chipped, the chips sprayed with insecticides, and the trees adjacent to the infected tree should be injected with propiconazole using a macroinfusion technique to prevent the spread of R. lauricola through root graphs (Ploetz et al., 2017a). The major limitations of the propiconazole-macroinfusion technique are that the procedure is labour intensive, expensive, and tree protection from disease is of limited duration and effective for not more than a year (Ploetz et al., 2017a). Repeated applications of systemic pesticides with stem injection systems can also be highly damaging to trees.

Gaps in Knowledge/Research Needs

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A better understanding is needed of the potential impacts of R. lauricola and X. glabratus on members of the Lauraceae in areas of the world where the pathogen and beetle are currently not present. This includes evaluations of the susceptibility of various tree and shrub species to laurel wilt caused by R. lauricola, and assessments of the ability of these species to serve as reproductive hosts for X. glabratus. Similarly, additional information is needed on the possibility that other bark and ambrosia beetles may acquire R. lauricola and transmit the pathogen to susceptible hosts. Very little information is currently available on the interaction of R. lauricola and X. glabratus with members of the Lauraceae and other plant families in Asia. Additional knowledge gaps include the mechanisms of resistance to laurel wilt in Asian lauraceous species as well as fundamental biological and ecological information on X. glabratus and its fungal symbionts in their native ecosystems.

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17/10/19 Original text by:

Stephen Fraedrich, US Forest Service, Athens, Georgia, USA

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