First Report of Laurel Wilt, Caused by Raffaelea lauricola, on Sassafras (Sassafras albidum) in Alabama

Abstract

Laurel wilt, caused by Raffaelea lauricola, a fungal symbiont of the redbay ambrosia beetle, Xyleborus glabratus, is responsible for extensive mortality of native redbays (Persea borbonia and P. palustris) in the coastal plains of the southeastern United States (1). The wilt also affects the more widespread sassafras, Sassafras albidum, particularly in areas where diseased redbays are common and populations of X. glabratus are high. Because sassafras stems were thought to lack chemicals that are attractive to the beetle, and sassafras tends to be widely scattered in forests, it was believed that the advance of the laurel wilt epidemic front might slow once it reached the edge of the natural range of redbay, which is restricted to the coastal plains of the Gulf and Atlantic Coasts (2). In July and August of 2011, wilt-like symptoms (i.e., wilted and dead leaves, and streaks of black discoloration in the xylem) were observed on 1 to 10 sassafras trees (15 to 23 cm diameter; 6 to 9 m height) at each of three locations, which were approximately 6 km from one another in Marengo Co., Alabama. Samples of the discolored wood from five trees were plated on malt agar amended with cycloheximide and streptomycin (CSMA), and a fungus morphologically identical to R. lauricola was isolated from each tree (1). For confirmation, a portion of the large subunit (28S) of the rDNA region of three of the isolates was sequenced (3); in each case, the sequence matched exactly that of other isolates of R. lauricola (EU123077) from the United States. Symptomatic trees were found at all three sites when revisited in April 2012, and approximately 20 sassafras trees in various stages of wilt were observed at one location, where only one diseased tree had been noted in 2011. Bolts were cut from the main stem of a symptomatic tree, and eggs, larvae, and adults of X. glabratus were commonly found in tunnels, and R. lauricola was isolated from the discolored xylem. Three container-grown sassafras saplings (mean height 193 cm, mean diameter 2.1 cm at groundline) were inoculated as previously described (1) with conidia (~600,000) from an isolate of R. lauricola. Three additional sassafras saplings were inoculated with sterile, deionized water, and all plants were placed in a growth chamber at 25°C with a 15-h photoperiod. Inoculated plants began to exhibit wilt symptoms within 14 days, and at 30 days all inoculated plants were dead and xylem discoloration was observed. Control plants appeared healthy and did not exhibit xylem discoloration. Pieces of sapwood from 15 cm above the inoculation points were plated on CSMA, and R. lauricola was recovered from all wilted plants but not from control plants. This is the first record of laurel wilt in Alabama and is significant because the disease appears to be spreading on sassafras in an area where redbays have not been recorded (see http://www.floraofalabama.org ). The nearest previously documented case of laurel wilt is on redbay and sassafras in Jackson Co., Mississippi (4), approximately 160 km to the south. The exact source of the introduction of X. glabratus and R. lauricola into Marengo Co. is not known. The vector may have been transported into the area with storms, moved with infested firewood, or shipped with infested timber by companies that supply mills in the area. References: (1) S. Fraedrich et al. Plant Dis. 92:215, 2008. (2) J. Hanula et al. Econ. Ent. 101:1276, 2008. (3) T. Harrington et al. Mycotaxon 111:337, 2010. (4) J. Riggins et al. Plant Dis. 95:1479, 2011.

Supercooling in the Redbay Ambrosia Beetle (Coleoptera: Curculionidae)

“…At the time of the Koch & Smith (2008) model, there was little evidence that X. glabratus would infest sassafras in the absence of redbay; since 2008 there has been mounting evidence to the contrary Smith et al 2009;Riggins et al 2011;Bates et al 2013). Consequently, the model only included areas where redbay and sassafras occur together.…”

“…As X. glabratus moves outside the range of redbay and swampbay, LWD has caused extensive sassafras mortality Smith et al 2009;Riggins et al 2011;Bates et al 2013). Disjunct populations are established in coastal counties of Mississippi and Alabama (Riggins et al 2010;Formby et al 2012), and in Marengo County, an interior county of Alabama (Bates et al 2013). Marengo County, Alabama is ~200 km from the nearest infestation site and exceeds a previous temporal estimation of invasion by 12 yr (Koch & Smith 2008).…”

“…In Duval County, Florida redbay mortality increased to 92% of mature trees within 2 yr of initial beetle infestations (Mayfield et al 2008b). As X. glabratus moves outside the range of redbay and swampbay, LWD has caused extensive sassafras mortality Smith et al 2009;Riggins et al 2011;Bates et al 2013). Disjunct populations are established in coastal counties of Mississippi and Alabama (Riggins et al 2010;Formby et al 2012), and in Marengo County, an interior county of Alabama (Bates et al 2013).…”

“…Currently, Marengo County is the only documented site where no redbay occurs and sassafras is the lone lauraceous species affected by laurel wilt. In 2012, more than 20 sassafras trees were reported in various stages of wilt, up from 1 tree the previous year (Bates et al 2013). Laurel wilt disease has not been documented infecting northern spicebush in the natural environment, but laboratory inoculation trials have shown it to be susceptible to the pathogen (Fraedrich et al 2006).…”

Laurel Wilt in Natural and Agricultural Ecosystems: Understanding the Drivers and Scales of Complex Pathosystems

“…The above findings have been on redbay, but sassafras has also been impacted. Bates et al [173] noted an outbreak on sassafras in Alabama, 160 km from the nearest outbreak, Fraedrich et al [174] described an outbreak in Louisiana, 400 km from the nearest outbreak, and Olatinwo et al [175] described a subsequent finding in Arkansas, 134 km north of the Louisiana outbreak. These range expansions suggest that anthropogenic spread is an important factor in the epidemiology of this disease.…”