California Forest Pest Council

Status: Introduced

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A systemic vascular wilt disease caused by the ophiostomatoid fungal pathogen Raffaelea lauricola and the redbay ambrosia beetle (Xyleborus glabratus), its primary vector.

Location: 

Southeastern US. Link to full distribution map here.

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Hosts:

Members of the Lauraceae family, including avocado (Persea americana). Severely impacted tree species native to the southeastern United States include redbay (Persea borbonia), silk bay (P. humilis), swamp bay (P. palustris), sassafras (Sassafras albidum), pondspice (Litsea aestivalis), and pondberry (Lindera melissifolia). Although the disease is currently not in California, artificial inoculation experiments confirmed that California bay laurel (Umbellularia californica) is highly susceptible (Fraedrich 2008). Other hosts not native to but present in California include avocado, camphortree (Cinnamomum camphora) and sweet bay (Laurus nobilis) (Smith et al. 2009; Hughes et al. 2014).

Management:

Management options to control laurel wilt are limited in the southeastern United States, where the beetle-pathogen has established and can no longer be eradicated. Thus, preventing its introduction into California is critical to protecting the 55,000 acres of avocado production and 1.2 million acres of hardwood and conifer forests that harbor California bay laurel throughout the state. Prevention involves controlling and containing the pest in the southeast, where it has spread across 11 states since 2002 (Olatinwo et al. 2021). Monitoring of sentinel trees along potential routes of introduction should also be a priority. Within California, prevention also entails risk assessments of priority sites and early detection monitoring to elicit a rapid response in the event of a novel introduction. Trapping is a useful tool for monitoring and should be used to detect the primary vector (X. glabratus) and potential alternate vectors (within Xyleborus and Xyleborinus), but is not an effective control treatment. The standard ethanol lures for ambrosia beetle detection can be used for trapping alternate vectors, but lures with an essential oil product enriched to 50% (–)-α-copaene content must be used to attract X. glabratus (Kendra et al. 2016). Consult your local county agricultural commissioner and forest health professional if you observe signs and symptoms on a suspect tree (see below). Because the beetle-pathogen can live in cut wood and human movement of firewood drives long-distance spread of laurel wilt (Ploetz et al. 2017), it is imperative not to move firewood over long distances. Instead, buy locally or gather on site where permitted. 

Early detection, rapid removal, and destruction of affected trees is the only measure proven to be effective at slowing disease spread in the southeast, especially in commercial avocado orchards. Stumps of felled infested trees are ground to soil level, and all wood is chipped to at least 1 square inch using sterilized equipment, followed by onsite burning, insecticide spray, or solarization for an extended time in a designated space (Crane et al. 2020). Prophylactic Tilt® injections (the fungicide propiconazole) have been shown to prevent laurel wilt outbreaks in commercial avocado groves (Crane et al. 2020), and could be an effective short-term measure on high-value trees or non-symptomatic hosts adjacent to infested trees. However, the treatment is very expensive, has a slow uptake rate, and needs to be reapplied every 12–24 months, depending on the dose, tree size, and onsite disease status at the time of injection (Crane et al. 2020, Olatinwo et al. 2021). Trenching or fungicide/herbicide treatments to sever root grafts may slow disease spread in avocado orchards where the pathogen can spread from tree to tree without vector assistance via interconnected root systems (Ploetz et al. 2017). Research is underway on treatment options involving host genetic resistance or tolerance and biocontrol agents (i.e., endophytic fungi and bacteria, extracts from the secondary metabolites of fungi, and entomopathogenic fungi).

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Highlights:

Laurel wilt disease is a pest-pathogen complex introduced into the southeastern United States from Southeast Asia. Since 2002, the redbay ambrosia beetle vector (Xyleborus glabratus) and fungal pathogen (Raffaelea lauricola) have killed an estimated 300 million redbay trees and other native lauraceous species in urban-wildland forests throughout the region (Ploetz et al. 2017). The disease has also significantly impacted the avocado industry in Florida. Growers can lose up to 15-20 trees per month (Crane et al. 2019), with up to $356 million estimated in potential economic losses (Evans et al. 2010). The complex is unique for two reasons: 1) unlike most ambrosia beetles, the redbay ambrosia beetle prefers to attack healthy, non-stressed trees; 2) R. lauricola aggressively moves systemically through the host xylem, unlike most ambrosia fungal symbionts, which cause local infections. As few as 100 conidia of R. lauricola can kill avocado and swamp bay (Hughes et al. 2015a). Tolerant host species, such as camphortree, could serve as symptomless reservoirs of the beetle-pathogen in the landscape and play a key role in disease persistence. Interestingly, R. lauricola has been horizontally transferred from the redbay ambrosia beetle to nine additional ambrosia beetle species since its introduction (Carrillo et al. 2014; Ploetz et al. 2017). These other species can also transmit the pathogen (Carrillo et al. 2014), suggesting potential for disease to spread among native and non-native beetle species if it were introduced in California. For avocado, the other Southeast native Xyleborus species (X. bispinatus and X. volvulus) are more important vectors of the pathogen than the redbay ambrosia beetle (Carrillo et al. 2014; Cruz et al. 2018, 2021, 2019; Saucedo-Carabez et al. 2018). 

In the early stages of the epidemic, adult female ambrosia beetles attack large healthy trees, attempting to bore natal galleries. Galleries are aborted, but trees are inoculated with the pathogen in these initial attacks, and the fungus spreads quickly throughout the tree’s vascular system (Fraedrich et al. 2008, 2015). Fungal propagules are also dispersed on other beetle species’ exoskeleton (Cruz et al. 2021), which may be sufficient for pathogen transmission. Once disease develops, infected host trees increase production of volatiles that attract more beetles, promoting a mass attack, colonization, and brood development within galleries of the host xylem where the developing colony feeds on the fungal symbiont cultivated in the gallery wall (Fraedrich et al. 2008, 2015). The host responds to attack in as little as three days before visible symptoms develop by producing tyloses and gels that block the xylem. Symptoms of discolored black sapwood and wilting green leaves that turn brown and persist on dying branches develop, and small holes with sawdust “toothpicks” are visible where the ambrosia beetles bore into the tree. Within a few weeks to several months, trees become choked of water and die. The pathogen can “silently” spread in avocado orchards where root-to-root transmission is common, with trees remaining asymptomatic until the epidemic is at the advanced stage. Males remain inside their natal galleries while mature adult females emerge carrying spores of R. lauricola in mycangia. The redbay ambrosia beetle uses host olfactory and visual cues in flight (not sex aggregation pheromones) to find a new host and produce their new gallery. Once laurel wilt establishes in a stand, most mature trees are killed within a few years.