Laurel Wilt

Management

The following section was extracted from Recovery plan for laurel wilt on redbay and other forest species. Caused by Raffaelea lauricola , vector Xyleborus glabratus ( Mayfield et al. 2009).

Limit transport of infested/infected host material
Do nothing
Sanitation
Chemical Control
Trap-Out or Attract and Kill
Germplasm Conservation
Discovery and utilization of resistant genotypes
Integrated Pest Management Strategy

Laurel wilt is well established, spreading rapidly, and killing redbay trees in large numbers within a geographic area that now encompasses roughly 60,000 square miles in Florida, Georgia and South Carolina, plus an outlying area in Mississippi. Eradication of the pathogen and the vector from the southeastern Atlantic Coast is not feasible. The following realities (and perhaps others) all suggest that high levels of host mortality due to laurel wilt will continue to occur through most of the redbay range, even under the best management efforts: the extreme virulence of the pathogen in redbay, the rapid speed at which it kills individual trees and spreads through a stand, the efficiency of the vector in finding and inoculating healthy hosts, the ability of one female beetle to initiate a new population of RAB without mating, the apparent lag time between arrival of the vector and detection of the disease in a new area, and the widespread distribution of redbay in a variety of habitats in the southeastern Atlantic Coastal Plain. A number of possible management strategies for dealing with laurel wilt in redbay and other forest species are discussed below:

Limit transport of infested/infected host material

Efforts to prevent the long-distance, human-assisted spread of the RAB and laurel wilt pathogen could help delay the onset of laurel wilt impacts in currently unaffected areas, possibly allowing more time for new management tactics to be developed. Limitations on long-distance dispersal could include:

• Restrict the transport of firewood, logs, mulch, and other unprocessed wood of redbay or other known hosts, out of counties (or other designated areas) in which laurel wilt is known to occur.
• Dispose of wood from killed redbays and other lauraceous hosts as locally as possible.
• Although movement of nursery plants is not yet a documented pathway by which laurel wilt has spread, periodically inspect field or container nurseries that sell lauraceous plants for laurel wilt symptoms and RAB infestations, particularly in areas where laurel wilt is known to occur. Shipment of stock from nurseries with the RAB would only be a problem if the beetle were in or on the nursery stock. Consider adopting an insect/disease-free certification program for lauraceous plants in nurseries prior to shipment. If the movement of infested nursery stock becomes a documented pathway of RAB, consider limiting the movement of nursery plants of known host species out of designated areas in which laurel wilt occurs.

Do nothing

In many forests and other natural areas with a redbay component, the most reasonable management response after laurel wilt is established may be to simply let the disease run its course.

Sanitation

As yet, there are no proven silvicultural or arboricultural treatments for mitigating the impact of laurel wilt. One attempt to eradicate the vector and pathogen from a local area through sanitation removal/destruction of RAB-infested trees was not successful. When laurel wilt was first detected on Jekyll Island, GA in 2006, the Georgia Forestry Commission (GFC) with cooperation from the Jekyll Island Authority deployed a chainsaw strike team and removed all known symptomatic redbays (more than 400 trees) from residential and natural areas and burned them at a central location. Nonetheless, by late 2007 laurel wilt was found throughout the island (James Johnson, GFC, unpublished presentation). This underfunded eradication attempt suggests that sanitation removal of only symptomatic host material is not an effective means of mitigating the impacts of laurel wilt during an epidemic. The tactic might be more fruitful if all host trees large enough for brood production by RAB are removed, if the sanitation is augmented by protection of individual trees with pesticides (see below), and/or if applied at a time when RAB populations, disease incidence, and host abundance in the landscape are very low. A solid commitment of funds and personnel to do follow-up surveys and complete removal of brood material in a timely fashion would be necessary for long-term success with sanitation. In addition, sanitation might be used to in conjunction with mass trapping (see below) to reduce RAB populations in defined areas. At the time of the Jekyll Island trial an effective lure had not been developed.

Although eradication attempts in the currently-infested areas of the southeastern Atlantic Coastal Plain appear fruitless, spread of the vector and pathogen into new areas might be slowed by prevention of long-distance spread by human activities, and possibly by rigorous, sanitation programs aimed at suppressing laurel wilt at outlier locations. Such sanitation efforts at or beyond the leading edge of the known laurel wilt range, however, would probably only be successful where host trees are relatively scattered and could be surveyed repeatedly throughout the year, where the disease and vector have not been present for very long, and where there is a full, long-term commitment (both financially and administratively) of involved agencies and stakeholders.

Although the pathogen has not been documented to spread by any means other than the beetle vector, arborists and tree-service professionals should consider cleaning/sterilizing saws and pruning blades after cutting an infected tree and before using them on uninfected lauraceous tree species. The potential for transmission of laurel wilt by pruning tools is currently being investigated by researchers at the University of Florida. Tree-service and utility pruning crews should be careful not to transport RAB-infested host material over long distances on chipping equipment or in chip vans.

Chemical Control

At one site in Florida, macro-infusion of redbay trees with the fungicide propiconazole was demonstrated to protect healthy redbay trees from development of laurel wilt after artificial inoculation with the pathogen (Mayfield et al. 2008a). Continued monitoring of trees in that study into 2009 suggests that the treatment, properly applied, can protect redbay trees for at least a year, but probably not for 2 or more years (A.E. Mayfield III, unpublished data). The macro-infusion process, which has also been used to protect against vascular wilt diseases in other species (e.g., oaks and elms), is relatively labor intensive and expensive, so it is impractical to implement on a large scale. It is currently unknown whether repeated use of this treatment can successfully protect a redbay tree through the duration of a laurel wilt epidemic, and if so, how many times a tree can or should be treated before the treatments can be discontinued (if at all). Recent data suggest RAB populations remain relatively high for 7-8 years following initial discovery until they fall to very low levels after the redbay population is gone (J.L. Hanula, unpublished). Treatment of symptomatic trees has not been evaluated and is not recommended. Nonetheless, propiconazole macro-infusion may be an option for protecting individual, high-value, landscape redbay trees, at least in the short-term. Removal of diseased redbay or other host material from the immediate area around the treated tree may help reduce the possibility of RAB attacks. Research may eventually demonstrate the efficacy of other fungicides, insecticides, or methods of delivery against RAB and the laurel wilt pathogen. Because RAB populations thus far have continued at very low levels even when all apparent host material is dead, individually treated trees may need to be protected in perpetuity.

Trap-Out or Attract and Kill

Manuka oil is as attractive to RAB as freshly cut redbay wood (Hanula and Sullivan 2008), and healthy, unwounded redbay trees are somewhat less attractive than wounded trees (Hanula et al. 2008). Although not yet demonstrated, protecting healthy trees by drawing RAB away from them may be possible. Combining manuka oil with other potential attractants in traps, or combining it with an insecticide and applying the combination to non-host substrates, may be effective ways to help minimize RAB populations within limited areas such as parks, neighborhoods or barrier islands. These techniques might be used to eradicate low-level RAB populations from isolated areas such as barrier islands after laurel wilt has eliminated most mature redbay trees. Mass trapping could be tested in combination with sanitation and chemical control to protect redbay trees that survive the initial wave of mortality.

Germplasm Conservation

Seed collection is routinely used as a means to preserve the genetic diversity of a plant species. Although seed of many species can be dried, stored below 0o C and remain viable for many years, long-term seed storage does not seem feasible for redbay. Due to their high lipid content, seeds of species in the Lauraceae are generally temperature recalcitrant, meaning it is difficult or impossible to sufficiently reduce the moisture content of the seed for storage at sub-freezing temperatures without killing the seed.

A protocol for collecting and submitting redbay seed to the USDA Forest Service National Seed Lab (NSL) in Dry Branch, GA was initiated in the summer of 2007 to determine the length of time redbay seed could be stored and to serve as a short-term storage facility for seed collected from impacted or potentially impacted areas. (see http://www.fs.fed.us/r8/foresthealth/laurelwilt/seed_collection/seed_collection.shtml).
Redbay seed from a wide variety of locations has been collected since 2007 and is being stored at the NSL. Using traditional methods, redbay seed can be stored for only a few years at 3o C with significant losses in viability each year.

Because long-term seed storage using traditional methods is not an option, other alternatives have been discussed. Cryopreservation of embryos from seeds has been identified as a possible option, but is expensive and labor intensive to implement on a large scale. Ex situ conservation of trees outside the southeastern Atlantic Coastal Plain region is another possibility, but that would require funding and a coordinated effort among organizations from across the region. More survey and impact data on species like sassafras, pondspice and pondberry would also be needed to help support and justify ex situ conservation programs for laurel wilt hosts. Because long-term germplasm preservation of these species is unlikely without an extensive outlay of money and resources, it would seem necessary to first demonstrate some likelihood of re-establishment success of host plant species into laurel wilt-impacted areas. The persistence of low-level RAB populations on places like Hilton Head Island (where the laurel wilt epidemic has already run its course) currently calls this likelihood into question.

Discovery and utilization of resistant genotypes

Jason Smith (University of Florida) and colleagues are investigating the possibility of natural resistance to laurel wilt in redbay populations. Research has included identifying and taking cuttings from putatively resistant redbays (mature trees that have remained alive in areas heavily impacted by laurel wilt), rooting them, and growing them to a size where they can be evaluated for disease susceptibility. Though laurel-wilt resistant redbays are not yet known, research may lead to their availability and use in the future.

Integrated Pest Management Strategy

An integrated management strategy is needed to limit the spread and impact of laurel wilt. This strategy may take various forms depending on the area of interest. In many forests and other natural areas with a redbay component, the most reasonable management response where laurel wilt is established may be to simply let the disease run its course. “Recovery” from laurel wilt in redbay and other forest species could be considered in terms of the following general courses of action:

• Slow the long distance, human-assisted spread of the disease.
• Improve our understanding of the biology, host associations, and impacts of the disease and its vector.
• Protect individual, high-value landscape trees with pesticides when feasible.
• Develop other tools for management of the disease and its vector, possibly to include sanitation, other silvicultural methods, trap-out or attract-and-kill techniques, use of resistant genotypes, and biological control.
• Assess the need for, and possibly pursue, a germplasm conservation program for threatened hosts.
• Continue to monitor the geographic spread of the disease, assess its impacts on host species as its spreads to new ecosystems, and educate the public about the issue.

Additional Information

• Mayfield, A.E. III, et al. 2009. Recovery plan for laurel wilt on redbay and other forest species. Caused by Raffaelea lauricola, vector Xyleborus glabratus. USDA National Plant Disease Recovery System. [462 KB] < http://www.ars.usda.gov/research/ >

• Cooperative Program to Manage X. glabratus in Avocado, Jorge Peña and Jonathan Crane, University of Florida, Laurel Wilt Conference 27-Feb-09 (Slideshow 990 KB).

• Fungicide Treatment of Redbays for Prevention of Laurel Wilt, Bud Mayfield, Florida Division of Forestry, Laurel Wilt Conference 27-Feb-09 (Slideshow 697 KB).

• Germplasm Conservation of Persea borbonia (L.) Spreng., Vic Vankus, US Forest Service, Cooperative Forestry, National Seed Laboratory, Laurel Wilt Conference 27-Feb-09 (Slideshow 1.8 MB).

• State Agencies: Regulatory Possibilities and Implementation, Wayne Dixon, Florida Department of Agriculture and Consumer Services, 27-Feb-09 (Slideshow 1.8 MB).

• Federal Regulation of the Laurel Wilt Epidemic, John Rogers, USDA APHIS PPQ, Laurel Wilt Conference 27-Feb-09 (Slideshow 64 KB).

• Mayfield, A.E. III, E.L. Barnard, J.A. Smith, S.C. Bernick, J.M. Eickwort, and T.J. Dreaden. 2008. Effect of propiconazole on laurel wilt disease development in redbay trees and on the pathogen in vitro. Arboriculture & Urban Forestry 35(5): 317-324. [PDF 228 KB]

• Redbay Ambrosia Beetle Suppression Efforts on Jekyll Island , Georgia , January 2007 - Chip Bates, Georgia Forestry Commission, 1/19/2007 (Abstract, 16 KB)

• Potential Treatments for Suppression of Xyleborus glabratus Infestation and Laurel Wilt in Persea borbonia (Redbay) - William E. Jones, Paul R. Merten, and Edwin K. Yockey, USDA Forest Service, 1/19/2007 (Abstract) Not currently available.

• Evaluation of Systemic Fungicides for Preventing Laurel Wilt - Albert E. Mayfield, III, FL Division of Forestry, 1/19/2007 (Abstract, 12 KB) (Slideshow, 192 KB)

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USDA Forest Service - Forest Health Protection, Southern Region
Last Modified: Friday, 17 May 2013 at 12:04:08 EDT