Taxonomic name: Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva 2008
Organism type: fungus
Raffaelea lauricola, a newly discovered fungus, causes laurel wilt in members of the Lauracae family, most notably redbay trees (Persea borbonia). It is vectored by Xyleborus glabratus an ambrosia beetle native to Asia. The recent introduction, rapid spread, and high mortality rates caused by R. lauricola and X. glabratus indicate the complex is a serious threat to the survival of redbay trees in southeastern United States. Furthermore, their invasive potential to the avocado industry in Florida and elsewhere could be devastating and extremely costly.
Condiophores of Raffaelea lauricola are single to aggregated in sporodochia, hyaline, unbranched or sparingly branched, one-celled to septate, and produce terminal conidia holoblastically. Conidiogenous cells proliferate percurrently or sympodially, leaving denticles, and lack inconspicuous scars or annelations. Conidia are small, hyaline, elliptical to ovoidal to globose, slimy, secession schizolytic, producing yeast-like growth through budding (Harrington et al, 2008).
agricultural areas, natural forests, planted forests
Very little is known about this newly discovered fungus. Raffaelea lauricola is known to inhabit tropical to temperate forests that bear suitable hosts of the Lauraceae family. It appears to only be vectored by X. glabratus and dependant on its climate range. One climate matching study estimated the native range climates of X. glabratus to possess an extreme minimum temperature range of -26-15°C and a 30-year mean growing season of 61% moisture (Koch & Smith, 2008).
Raffaelea lauricola is a very aggressive vascular wilt pathogen that causes laurel wilt in members of the Lauraceae family, most notably redbay trees (Persea borbonia). It is capable of colonizing the entire tree from a single introduction into small branches or stems by egg-laying attacks of the redbay ambrosia beetle (Xyleborus glabratus). When the beetle introduces R. lauricola into trees the fungus spreads through the vascular system of trees, blocking water transport, and causes the trees to wilt and die within months of initial infection. The wood of dying and recently killed trees serves as host material for X. glabratus and other ambrosia beetles, which multiply rapidly, resulting in large numbers of beetles capable of spreading to new locations. The ambrosia beetle has a rapid rate of natural spread that has been estimated at 34 miles/year and may be advanced further by anthropogenic dispersal of infested wood products. Currently X. glabratus is the only confirmed vector of R. lauricola.
The redbay ambrosia beetle and its accompanying R. lauricola have spread the wilt through forests of coastal southeastern United States, killing almost all redbays over 2.5 cm in diameter in its path. A survey of redbays in counties near Savannah, the point of introduction of X. glabratus and R. lauricola in Georgia, found mortalities of redbays as high as 70% to 90% in the most heavily infested locations. Other known, susceptible hosts include: sassafras (Sassafras albidum), silkbay (Persea borbonia humilis, California bay (Umbellularia californica), avocado (Persea americana), pondspice (Litsea aestivalis), which is endangered in Florida and threatened in Georgia, and pondberry (Lindera melissifolia), which is federally endangered. Additionally R. lauricola has been isolated from camphor (Cinnamomum camphora) trees with branch dieback. However the collapsing wilt seen in redbay and other species has not been observed (Smith et al., 2009).
Laurel wilt has essentially eliminated mature redbay trees from a large portion of the South Atlantic Coastal Plain and maritime forests. R. lauricola and X. glabratus may continue to expand throughout coastal plain forests from Virginia to Texas. This invasive complex is considered a “very high risk”. R. lauricola and the ambrosia beetle are also a major threat to the avocado industry in south Florida, Mexico, and elsewhere. The invasion of Florida’s avocado groves, valued around $13-30 million annually, by X. glabratus and R. lauricola could be devastating.
Redbay is important to wildlife as its fruit, seed and or foliage are eaten by several species of songbirds, wild turkeys, quail, deer, and black bear, particularly in winter foraging. Two butterflies, the spicebush swallowtail (Papilio troilus) and the Palamedes swallowtail (P. palamedes) use Lauraceae species as hosts. Although P. palamedes larvae may feed on redbay or sassafras, adult females strongly prefer to oviposit on redbay. Reduction in redbay densities may have significant negative impacts on P. palamedes populations in their current range (Mayfield & Thomas, 2006; Koch & Smith, 2008).
R. lauricola and X. glabratus are thought capable of spreading throughout the entire range of redbays in the U.S. in less than 40 years. It may eventually become even more widely distributed if it continues to spread in sassafras, which is found in much of the eastern half of the U. S. There are many additional genera and species in the laurel family, concentrated mainly in the tropical and subtropical areas in Central and South America, which may also be susceptible (Cameron et al., 2008; Koch & Smith, 2008).
Indications of trees infested by the ambrosia beetle and laurel wilt include beetle entrance holes, cylinders of frass emanating from the trunk, black discoloration of sapwood that start as streaks and becomes extensive throughout cross sections, wilting of branch tips and leaves, dieback of individual branches, and red and brown-to-purple coloration of dying leaves that persist over a year after its death (Fraedrich et al., 2008; Hoddle, 2009).
The association of a wilt-causing fungus with an ambrosia beetle is unprecedented. The only previously documented case of extensive mortality of forest trees caused by an ambrosia beetle and a fungal symbiont appears to be the death of oaks Quercus crispula and Q. serrata in Japan associated with Raffaelea quercivora, a fungal associate of the ambrosia beetle Platypus quercivorus. This ambrosia beetle is thought to be indigenous to Asia and the decline of oaks in Japan has been documented since the 1930s. The attacked trees appear to be weakened by drought or other agents, and mass attacks by P. quercivorus are necessary for the oak trees to die. R. quercivora is thought to facilitate tree death through localized colonization of the tree in the vicinity of the ambrosia beetle galleries, but the fungus does not colonize the host systemically. In contrast, the Raffaelea lauricola that causes laurel wilt is a very aggressive vascular wilt pathogen that is capable of colonizing the entire tree from a single introduction into small branches or stems by egg-laying attacks of X. glabratus (Fraedrich et al., 2008; Cameron et al., 2008).
Native range: India, Japan, Taiwan, Burma, Bangladesh, Myanmar
Known Introduced range: Southeastern United States
Introduction pathways to new locations
Solid wood packing material: Raffaelea lauricola is readily isolated from the mycangia of the exotic beetle Xyleborus glabratus and assumed brought to the United States from Asia with its vector in solid wood packing material (Fraedrich et al. 2008).
Local dispersal methods
Preventative measures: The Laurel Wilt Working Group public education and outreach campaigns recommended that emphasis be placed on the potential destructive consequences of moving firewood and raw materials infested with Raffaelea lauricola and Xyleborus glabratus to uninfested, areas and those groups of people most likely to transport infested host material should be targeted (Cameron et al., 2008).
Researchers have proposed that redbays be removed from a 4 to 5 km thick band surrounding the known populations of X. glabratus. Such wide areas devoid of redbay hosts may deter the spread of R. lauricola by its vector X. glabratus. A similar measure has been suggested around avocado production areas in south Florida by the Laurel Wilt Working Group (Cameron et al, 2008; Hess, 2007; Hanula et al, 2008a).
Avocado farmers are advised to be extremely vigilant regarding R. lauricola and X. glabratus. They are encouraged to report signs of wilt and dieback in the United States to the Florida Division of Plant Industry. They are also advised to avoid plant stress on crops as it makes them more vulnerable, and to not move infested crops but burn the individual crops in the grove (Crane, 2009).
Chemical control: The fungicide propiconazole may be a useful method of preventing infection of redbay trees by R. lauricola. Research results found that propiconazole completely inhibited mycelial growth of Raffaelea spp. in vitro at concentrations 0.1 parts per million (ppm) or greater and was fungitoxic at 1 ppm or greater. Ten mature redbay trees received root-flare injections of propiconazole and none developed crown wilt symptoms for at least 30 weeks after being inoculated with Raffaelea spp., whereas nine of ten untreated control trees wilted in more than one-third of their crowns. Concentrations of propiconazole diminish in trees over time but the fungicide was retained in the stem xylem for at least 7.5 months after injection. Fungicide injections may not be a viable technique for avocado crops because at this time there are no fungicides registered for use on avocado. Control of the redbay ambrosia beetle is complicated by the fact that adult beetles must be in the immediate area of aerial sprays to be controlled. Detection of adult beetles involves monitoring traps and/or scouting groves. Once adult beetles bore into the trees contact insecticides are ineffective. The current recommendation is to periodically apply permethrin to the trunk and major limbs of uninfested, potential host trees up to about 10 ft above the ground beginning in early March. If an infestation is taking place, an application of permethrin is recommended to knock down the population as soon as possible (Mayfield et al., 2008a; Crane, 2009).
Raffaelea lauricola derives its nutrition from its host. Suitable hosts include members of the Lauraceae family including redbay (Persea borbonia), sassafras (Sassafras albidum), camphor (Cinnamomum camphora), silkbay (Persea borbonia humilis, California bay (Umbellularia californica), avocado (Persea americana), pondspice (Litsea aestivalis), and pondberry (Lindera melissifolia) (Cameron et al, 2008; Fraedrich et al, 2008)
Raffaelea lauricola is an asexual fungus that typically produces small conidiophores in tight clusters (sporodochia) in beetle galleries. The fungus is introduced into its host from the mycangia of its ambrosia beetle vector Xyleborus glabratus (Harrington et al., 2008).
A laboratory culture of Raffaelea lauricola provided the following evaluation: "Colony at 10 days cream-buff, smooth, but later mucilaginous in the center, margins of colony uneven, side branches of submerged hyphae at advancing front producing conidia and tight clusters of blastoconidia; 2 week old colonies cottony, honey yellow, and with a yeasty odor. Conidiophores hyaline, usually aseptate and unbranched but sometimes septate at branches, terminal or arising as a side branch from hyphae, variable in length but mostly (8.5)13-60(120) x (1.0)2.0(2.5) .tm wide. Conidia produced holoblastically, at the tip of the conidiogenous cell, but not leaving conspicuous scars or annelations, primary conidia oblong to obovoid,sometimes flattened at the point of attachment, hyaline, thin walled, (3.0)3.5-4.5(8.0) x (1.0)1.5-2.0(3.5) tun; budding new cells, the blastospores forming in a cluster at the tip of the conidiophore and a slimy mass over the central part of the colony" (Harrington et al, 2008).
Reviewed by: Stephen W. Fraedrich, Insects, Diseases, and Invasive Plants, USDA Forest Service
Principal sources: Cameron, R. Scott; Chip Bates and James Johnson., 2008. Distribution and Spread of Laurel Wilt Disease in Georgia: 2006-08 Survey and Field Observations . Georgia Forestry Commission September 2008 Funded partially by the USDA Forest Service Forest Health Protection Region 8
Fraedrich, S. W; Harrington, T. C; Rabaglia, R. J; Ulyshen, M. D; Mayfield, A. E. III; Hanula, J. L; Eickwort, J. M; Miller, D. R., 2008. A fungal symbiont of the redbay ambrosia beetle causes a lethal wilt in redbay and other Lauraceae in the southeastern United States Plant Disease. 92(2). FEB 2008. 215-224.
Harrington, T.C., Fraedrich, S.W., Aghayeva, D.N. 2008. Raffaelea lauricola, a new ambrosia beetle symbiont and pathogen on the Lauraceae. Mycotaxon, Volume 104, pp. 399-404 (April - June 2008).
Compiled by: National Biological Information Infratsructure (NBII) & IUCN SSC Invasive Species Specialist Group (ISSG)
Last Modified: Monday, 7 June 2010