Friday, August 7, 2009

PS 88-138: Predictions and early observations on the impact of Laurel Wilt Disease on the flora of North America

Joel M. Gramling, The Citadel


Laurel Wilt Disease is a plant disease that affects species in the Lauraceae.  Laurel Wilt Disease occurs when the fungal pathogen Raffaelea lauricola is introduced to Lauraceous species in the southeastern United States via an Asian ambrosia beetle, Xyleborus glabratus.  The effects of this disease were first observed on Hilton Head, SC in 2004 and are thought to be the result of the introduction of the non-native X. glabratus at Port Wentworth near Savannah, GA.  As of February 2009, Laurel Wilt Disease has been observed in 11 counties in southeastern South Carolina, 22 counties in coastal Georgia and 18 counties in northern and eastern Florida.  To assess the potential impact of Laurel Wilt Disease a list of all plant species in the Lauraceae that occur in North America (north of Mexico) was compiled that included the conservation status of each species, associated plant communities and whether each species has been observed to exhibit mortality related to the disease in the lab or in the field.  Monitoring plots were established across coastal South Carolina to quantify the effects of the disease on actual plant populations.  These plots were used to assess trends in morality and resprout activity associated with the disease.


Twelve native and three introduced species in the Lauraceae are thought to be at risk of being impacted by Laurel Wilt Disease.  At least four of these native species are already viewed as vulnerable to extirpation or extinction.  Over 55 native plant communities recorded in the International Vegetation Classification system found in the United States and Canada may be impacted by Laurel Wilt Disease.  Initial observations from the monitoring plots have demonstrated that mortality associated with Laurel Wilt Disease is greatest in large-stemmed individuals (DBH > 10cm) initially.  Similarly, over time stems < 2.5 cm in diameter were significantly less likely to exhibit the disease.  A die-back of the primary stem due to Laurel Wilt Disease was found to correlate with a doubling in the average number of resprouts growing from the base of the stem.  Overall initial observations suggest that Laurel Wilt Disease preferentially affects large stems, exhibits a high mortality rate, and spreads through the stem rapidly, but may not invade the root system.