Xyleborus glabratus attacks and systemic colonization by Raffaelea lauricola associated with dieback of Cinnamomum camphora in the southeastern United States

Abstract

Laurel wilt, caused by Raffaelea lauricola, is responsible for extensive mortality of redbay and other American members of the Lauraceae in the southeastern United States. Raffaelea lauricola is a mycangial symbiont of the redbay ambrosia beetle (Xyleborus glabratus), and the beetle and fungus were accidentally introduced from Asia. Branch dieback of camphortree (Cinnamomum camphora), an Asian member of the Lauraceae, has been occasionally observed in areas where laurel wilt has decimated redbay populations, and R. lauricola was isolated from such camphortrees. However, the role of X. glabratus and R. lauricola in this branch dieback remains unclear. Examination of camphortrees on Jekyll Island, Georgia showed that healthy-appearing trees and those with branch dieback had been attacked by X. glabratus, but the trees with branch dieback had four times as many beetle attacks. Raffaelea lauricola was routinely isolated from discoloured xylem near beetle tunnels in healthy trees and those with dieback. Single-point inoculations with R. lauricola on stems of mature, healthy camphortree trees failed to induce wilt-like symptoms or branch dieback, although areas of discoloration were scattered throughout the xylem, and R. lauricola was reisolated irregularly at various heights in some inoculated trees. In growth chamber experiments, single-point inoculations with R. lauricola resulted in systemic colonization but no wilt symptoms or branch dieback in camphortree saplings. In contrast, inoculations at multiple points along the stem (simulating multiple attacks by the vector) caused branch dieback and wilt-like symptoms, including a brownish, diffuse discoloration of the xylem. Camphortree appears to be more resistant than American species of Lauraceae to the vascular wilt caused by R. lauricola. The fungus does colonize camphortrees systemically, however, and can apparently cause branch dieback. This suggests that the fungus may provide brood material for X. glabratus in Asia as it does in the southeastern United States.

Plasticity of mycangia in Xylosandrus ambrosia beetles

“…The beetles transport specific fungal symbionts from their natal galleries to newly established galleries in a storage organ termed a mycangium (Batra, 1963;Francke-Grosmann, 1963), but both can be kept in the laboratory on artificial media (Biedermann et al, 2009). A third reason is their importance: In recent years, several ambrosia beetle-fungus symbioses have developed outbreaks causing significant economic and ecological damages, and understanding the interactions between the beetle and the fungus has immediate scientific, economic and ecological implications (Fraedrich et al, 2008;Hulcr & Dunn, 2011;Eskalen et al, 2013;Fraedrich et al, 2015;.…”

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

“…They develop vascular symptoms, but do not die, which is similar to the response of camphortree [33]. Notably, rare tolerance to laurel wilt has been selected in redbay, and there is hope that these selections could be used to re-establish this tree in the southeastern coastal plain [36,120].…”

“…During the early stages of the laurel wilt epidemic, Fraedrich et al [13,33] examined X. glabratus x redbay interactions. They indicated that initial attacks by the insect in healthy trees were aborted and that reproduction by the insect was not observed in such trees.…”

“…Nonetheless, aborted attacks were sufficient to infect trees with R. lauricola. Only after laurel wilt began to develop in infected trees was brood development by X. glabratus observed [13,33]. If this sequence is typical, X. glabratus may resemble other ambrosia beetle species in that it preferentially colonizes and reproduces in compromised or dead trees.…”

“…To date, we have only rudimentary understandings of these processes [33,[121][122][123]. Although natural selection against susceptibility has probably occurred in camphortree and other Asian Lauraceae [1,33], host attributes that are associated with laurel wilt tolerance have been studied only recently [121].…”