Laurel wilt, also called laurel wilt disease, is a vascular disease of the Lauraceous plant caused by the fungus Raffaelea lauricola[1] and transmitted by the invasive redbay ambrosia beetle, Xyleborus glabratus. The disease affects and kills members of the laurel family. The avocado is perhaps the most commercially-valuable plant affected by laurel wilt.
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Symptoms of laurel wilt include wilted stems and leaves, black streaking in the wood, and strings of compacted sawdust protruding from tree trunks.[2] Laurel wilt can spread in at least two ways: one is via the beetle's natural reproduction and migration. A second way is through the sale and transport of beetle-infested wood, a result of redbay's use as firewood and for outdoor grilling.[2]
Laurel wilt has been found in South Carolina and Georgia, and notably in Florida, where it has reached as far south as Okeechobee County and as far west as Columbia County.[2] The redbay ambrosia beetle was discovered in Savannah, Georgia's Port Wentworth area in spring 2002; however, it is likely to have been established in the area prior to 2002 when the three adult specimens were trapped at the port. The beetle likely entered the country in solid wood packing material with cargo that was imported at Port Wentworth. Redbay trees began dying in Georgia and South Carolina near the Savannah area in 2003. By early 2005, officials with the Georgia Forestry Commission (GFC), South Carolina Forestry Commission (SCFC), and USDA Forest Service began to suspect the newly discovered ambrosia beetle was associated with this mortality.[3]
The redbay (Persea borbonia), a tree particularly abundant in maritime forests of Georgia, South Carolina and Florida, has been the primary species affected by the wilt. Sassafras, a less common tree in the coastal plains of the Southeast but with a more extensive range than redbay, has also been affected by the disease but to a lesser extent than redbay. The wilt fungus has also been isolated from dead and dying pondspice (Litsea aestivalis) and pondberry (Lindera melissaefolia), however the redbay ambrosia beetle has not been found in either of these species. Pondberry is a federally endangered species while pondspice is regarded as a threatened or endangered species in some southeastern states.[4]
The state’s avocado crop earns about $30 million wholesale each year, with commercial avocados growing on 7,500 acres (30 km2) mostly in Miami-Dade County. The avocado crop accounts for more than 60 percent of Florida’s tropical fruit production. 60,000 Florida residents also have at least one avocado tree in their yards.[2] Some estimates project that the disease could cut avocado crop in half, at a total economic impact of about $27 million.[2]
In a September 2008 study, a possible fungicide was tested. The abstract of the study reads as follows:
In this study, the systemic fungicide 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, whereas the fungicide thiabendazole was less inhibitory. None of the ten mature redbay trees that received root-flare injections of propiconazole 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. Propiconazole was retained in the stem xylem for at least 7.5 months after injection but was more frequently detected in samples from trees injected 4.5 months earlier and was not well detected in small-diameter branches. Results suggest that propiconazole may be useful in preventing laurel wilt in redbay, but limitations and questions regarding duration of efficacy, rate of uptake, and efficacy under different levels of disease pressure remain.[5]
It is important to note that while this fungicide may be useful in protecting some trees against the wilt, this fungicide is NOT cleared for use on fruit trees that are part of commercial production for human consumption.
Some redbay trees may be resistant to the disease, and future research will investigate factors associated with resistance, in the hope that tolerant varieties can be identified and developed.[2]