Progression and Impact of Laurel Wilt Disease within Redbay and Sassafras Populations in Southeast Georgia
Laurel wilt disease (LWD), caused by the fungus <i>Raffaelea lauricola</i> and transmitted by <i>Xyleborus glabratus</i> (Redbay Ambrosia Beetle [RAB]), has killed millions of <i>Persea borbonia</i> (Redbay) trees throughout the southeastern Coastal Plain. Laurel wilt also has been detected in <i>Sassafras albidum</i> (Sassafras) in widely dispersed locations across the southeastern US. We established long-term laurel wilt disease-progression plots in Redbay
and Sassafras stands in southeastern Georgia and monitored them through 4 years to document mortality rates and investigate long-term effects of LWD on Redbay and Sassafras survival and regeneration. Laurel wilt disease killed 87.3% of Redbay and 79.5% of Sassafras trees in the plots. The time from initial LWD detection to inactivity (no new mortality) in Redbay stands ranged from 1.1 to 3.6 years, with rate of disease progression positively related to host-tree size and abundance. Larger trees died at a higher rate in both Redbay and Sassafras stands, and mortality curves were similar for both species. All diseased Redbay trees died to the ground level, but the majority produced persistent below-ground basal sprouts, rapidly providing potential replacement stems. Few below-ground basal sprouts were observed on Sassafras trees killed by LWD, but over a quarter had epicormic shoots that survived up to several years after infection, and small trees remained alive on most sites, suggesting some level of tolerance to LWD. Substantial numbers of RAB were only captured in baited traps located adjacent to plots in an advanced-active stage of disease progression with
abundant infested trees, both in Redbay and Sassafras stands. However, lingering presence of small numbers of RAB in post-epidemic areas and scattered LWD mortality in small-sized Redbay regeneration sprouts and seedlings suggest that secondary disease cycles may occur as Redbay trees there reach greater numbers and size in the future. Documentation of RAB and LWD spreading in Sassafras in the absence of Redbay supports concern that LWD will continue to spread into areas with abundant, large Sassafras trees, which would increase the probability that RAB and LWD will expand into extensive populations of other laurel species present in the western US and Central and South America.
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