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Laurel wilt

Scientific Name
Raffaelea lauricola
Scientific Author
T.C. Harr., Fraedrich & Aghayeva
Taxonomy
Ascomycota, Sordariomycetes, Ophiostomatales, Ophiostomataceae
Status
Exotic species - absent from Australia
Reliability
High
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Diagnostic Notes
Laurel wilt is caused by the fungus Raffaelea lauricola, which is vectored by the ambrosia beetle Xyleborus glabratus. X. glabratus has a symbiotic relationship with R. lauricola: female beetles bore into the sapwood of trees to create brood galleries and at the same time inoculate the tree with the fungus. The beetle larvae then feed off the fungal hyphae as it colonizes the wood.

The severity of symptoms of laurel wilt depends on the host species. Highly susceptible hosts such as redbay (Persea borbonia), swampbay (Persea palustris) and West Indian races and hybrids of avocado (Persea americana) wilt rapidly and the entire tree declines and dies (Fraedrich et al. 2007; Mayfield et al. 2008). In other more tolerant hosts, for example camphor laurel (Cinnamomum camphora), wilting is localized and dieback is restricted to individual branches or infection is asymptomatic (Smith et al. 2009b). As a generalization, lauraceous species from South East Asia (where the fungus originates from) are more likely to be tolerant of infection than those from North America (R. Ploetz, personal communication).

In the early stages of infection, the leaves of Persea spp. droop and take on a reddish or purplish tinge. These leaves turn brown as the branch dies, and wilting happens so rapidly that an abscission layer does not form and the dead leaves are retained on the branch for as long as 12 months. In contrast to Persea spp., diseased sassafras (Sassafras albidum) rapidly defoliates as the tree wilts and dies (Smith et al. 2009a).

Upon removal of the bark from a diseased branch, small entrance holes and tunnels of the beetle are evident, from which brownish-black stains extend. When the sap wood is cut, the diseased vascular bundles appear as dark streaks. Tubes of frass (sawdust) may protrude from the entrance holes and powdered frass may accumulate in forks of the tree or at the base of the trunk.

R. lauricola and X. glabratus are intimately associated, and wherever one is found, the other will almost certainly be found. However, six different Raffaelea species have been isolated from X. glabratus in the USA, although R. lauricola was the most abundant and isolated from all but one individual that was tested (Harrington et al. 2010a; Harrington and Fraedrich, 2010b). The fungus can be isolated from both surface-sterilised wood chips and beetle heads using a selective medium containing cycloheximide and streptomycin sulphate (Harrington and Fraedrich, 2010b).

The fungus

The following description of R. lauricola is extracted from Harrington et al. (2008).

Maximum growth rate on malt agar at 25ºC, colony diameter 60 mm at 25ºC, 10 mm diameter or less at 10 and 30ºC. Colony at 10 days cream-buff, smooth, but later mucilaginous in the centre, 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 odour. 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) × (1.0)2.0(2.5) µm wide. Conidia produced holoblastically, at the tip of the conidiogenous cell, but not leaving conspicuous scars or annelations, primary conida oblong to obovoid, sometimes flattened at the point of attachment, hyaline, thin walled, (3.0)3.5-4.5(8.0) × (1.0)1.5-2.0(3.5) µm; 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.
More Information
Specimen Contact
Andrew Geering
Author
Andrew Geering, Paul Campbell
Created
27/04/2011 03:47 AEST
Last Updated
27/05/2011 17:08 AEST
Citation
Andrew Geering, Paul Campbell (2011) Laurel wilt (Raffaelea lauricola) Updated on 5/27/2011 5:08:22 PM Available online: PaDIL - http://www.padil.gov.au.
Image Use
Free for use under the Creative Commons Attribution 3.0 Australia License

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