R. lauricola has been described in detail by Harrington et al. (2008). Optimal colony growth of R. lauricola occurs at 25°C, and cream-buff and smooth colonies develop on malt extract agar that are approximately 60 mm diameter after 10 days (Harrington et al., 2008). Colonies tend to become mucilaginous in their centres and these areas are dominated by budding yeast-like conidia. Colonies that develop from spores tend to be mucilaginous initially and after several days submerged hyphae develop at the colony margins. Conidiophores are hyaline, typically aseptate, and unbranched with lengths variable, usually ranging from 13-60 µm (range 13-120 µm) and 2 µm wide (range 1-2.5 µm). The conidia are hyaline and small, typically 3.5-4.5 µm (range 3.0-8.0 µm) x 1.5-2.0 µm (range 1.0-3.5 µm) and varying from elliptical to ovoid to globose (Harrington et al., 2008).
The detection of laurel wilt in redbay and sassafras is usually straightforward with wilted and dead foliage occurring in some branches initially and eventually over the entire crowns of trees (Fraedrich et al., 2008). A black discolouration is observed in the sapwood of stems and branches. Initially, the discolouration is primarily evident in the outermost sapwood but as the disease progresses the discolouration will be observed through much of the cross-sectional area of the sapwood. Isolation of the pathogen from infected tissues on agar media is necessary to confirm the disease diagnosis. Frass tubes are typically observed on stems and branches of redbay and other species being attacked by X. glabratus, and frequently these are numerous after trees have wilted.
Related invasive species
- Raffaelea lauricola
Related Farm Practice
- Hosts
- Sapwood
- Development
Laurel wilt is responsible for the death of hundreds of millions of redbay (Persea borbonia sensu lato) trees throughout the southeastern USA, and the disease is also having significant effects on other species such as sassafras (Sassafras albidum) in natural ecosystems and avocado (Persea americana) in commercial production areas of south Florida. Laurel wilt is caused by the pathogen Raffaelea lauricola, a fungal symbiont of the redbay ambrosia beetle, Xyleborus glabratus. Thus far, the disease is confined to members of the Lauraceae that are native to the USA, or native to such places as the Caribbean, Central America and Europe and grown in the USA. The beetle and fungus are native to Asia and were likely introduced with untreated solid wood packing material at Port Wentworth, Georgia in the early 2000s. Since that time laurel wilt has spread rapidly in the coastal plains of the southeastern USA, spreading north into central North Carolina, as far west as Texas, and reaching the southernmost counties of Florida. Current models suggest that X. glabratus can tolerate temperature conditions that occur throughout much of the eastern USA, and so the disease threatens sassafras throughout much of this region. The disease poses a threat to lauraceous species indigenous to other areas of the Americas as well as Europe and Africa.
R. lauricola moves rapidly in the xylem of trees (Fraedrich et al., 2015a) and disease symptoms are often observed in portions of redbay trees within two to four weeks after infection. The disease then spreads throughout the entire crown, and redbay trees typically wilt completely within 4 to 12 weeks following inoculation (Fraedrich et al., 2008;Mayfield et al., 2008b). Leaves of infected trees initially droop from loss of turgor and then turn a reddish-brown colour as they die. Some older leaves may initially become chlorotic as they are dying. Leaves on redbay and some other evergreen hosts do not abscise after dying and can be retained on branches for a year or more after the tree has died. A dark black discolouration is observed in the stem and branch sapwood of infected plants. The discolouration is initially observed in the outermost sapwood as localized streaks in the early stages of wilt, but over time the discolouration occurs more extensively throughout the cross-sectional area of the xylem tissue. Symptom development is similar in sassafras and other deciduous hosts except leaves are likely to drop as they die or soon after (Fraedrich et al., 2008;Fraedrich et al., 2015a). Sassafras leaves take on a reddish discolouration (Fraedrich et al., 2008) and pondberry leaves become very chlorotic and turn a bright yellow as they die (Best and Fraedrich, 2018).
The rate of development of the disease and subsequent symptoms in redbay plants depends greatly on their size and environmental factors, such as temperature and moisture conditions. The disease appears to progress relatively slow in trees infected late in the growing season, and trees with partial crown wilt on only a few branches can be found during the winter months. The disease progresses much more rapidly, and trees die quickly, during the spring and summer months when trees are actively transpiring and growing.
Symptom development in avocado is somewhat similar to that of redbay. The first symptom is the wilting of terminal leaves and the development of brown-to-black discolouration as they die (Ploetz et al., 2017a). Unlike redbay, leaves of avocado tend to abscise within 2 to 9 months following symptom development (Ploetz et al., 2017a). Apparently symptoms can be localized in avocado trees with the disease affecting some branches in portions of trees, and epicormic sprouting beneath the affected portions of trees can subsequently lead to the production of healthy branches (Ploetz et al., 2017a). The sapwood of infected trees develops a brown to black discolouration (Mayfield et al., 2008c).
Many members of the Lauraceae that are native to the southeastern USA appear to be highly susceptible to laurel wilt, although some have not been greatly impacted by the wilt for various reasons. A couple of species in the Lauraceae that are native to Florida appear to be somewhat resistant to the disease. Species indigenous to South East Asia appear to be mostly resistant to laurel wilt. A more complete assessment of what is known about the susceptibility of the individual species follows.
Persea borbonia (redbay) and P. palustris (swampbay) are very similar taxa with differentiating characteristics that are vague and not always reliable (Coker and Toten, 1945). Some have regarded the two taxa as the same species or varieties of a species (Radford et al., 1968;Little, 1979) while others consider them to be separate species (Shearman et al., 2018;Weakley 2015). Regardless, redbay and swampbay, historically treated by many as one species (Radford et al., 1968;Brendemuehl, 1990), appear to be equally susceptible to laurel wilt (Fraedrich et al., 2008), and are difficult to accurately distinguish under field conditions. Thus, redbay is treated in this database as a single species (i.e. P. borbonia sensu lato). Redbays are evergreen trees that occur in the coastal plain forests of the southeastern USA, and are a minor use hardwood and ecologically important in the forest ecosystems where they occur (Brendemuehl, 1990). Redbay has been affected by laurel wilt throughout much of its range with losses that are estimated into the hundreds of millions of trees (Hughes et al., 2017). The disease preferentially affects larger diameter trees (Fraedrich et al., 2008;Mayfield and Brownie, 2013) and throughout its range there is still high survivorship among smaller diameter redbay trees as well as sapling and seedlings (Cameron et al., 2015). The reason for this phenomenon is thought to be due to the preference of X. glabratus to attack larger diameter trees (Mayfield and Brownie, 2013) and not due to resistance in the smaller diameter plants (Fraedrich et al., 2008). Thus, although redbay trees have been devastated by laurel wilt in the southeastern USA, it does not appear that the species is in imminent danger of extinction. The long term survival of redbay in the southeastern USA will depend on the ability of X. glabratus to find and reproduce in smaller diameter redbays or other suitable hosts.
Sassafras albidum (sassafras) is a deciduous tree species that occurs in various forest types over much of the eastern half of the USA (Griggs, 1990;Randolph, 2017) and is a minor use hardwood (Harding et al., 1997;Cassen, 2007). Pathogenicity tests confirmed that sassafras is highly susceptible to laurel wilt (Fraedrich et al., 2008) and the disease has affected sassafras across the southeastern USA (Bates et al., 2013;Fraedrich et al., 2015a, Olatinwo et al., 2016). Recent studies indicate that X. glabratus can survive the low winter temperatures throughout much of the range of sassafras (Formby et al., 2018), however at this time, the northern most location of the disease is central North Carolina (Mayfield et al., 2019).
Persea americana (avocado) is a tropical evergreen tree that is native to Central America and the Caribbean. The species is cultivated for production of avocados in Florida and California in the USA, and is also grown in Mexico and many other countries. Three distinct races of avocado are recognized that include the Mexican, Guatemalan and West Indian. The West Indian and West Indian-Guatemalan hybrids are primarily cultivated for commercial production in Florida (Mayfield et al., 2008a). Some avocado cultivars are more susceptible to laurel wilt than others, and West Indian cultivars such as ‘Simmonds’ appear to be highly susceptible (Mayfield et al., 2008a;Ploetz et al., 2012a). The ‘Simmonds’ cultivar comprises approximately 35% of the avocado production in Florida (Ploetz et al., 2011b). The West Indian-Guatemalan hybrids are generally susceptible but less so than the West Indian cultivars, and Guatemalan x Mexican hybrids such as the ‘Hass’ cultivar appear to be among the most resistant (Mayfield et al., 2008a;Ploetz et al., 2012a). The ‘Hass’ cultivar accounts for 95% of all production in California (Ploetz et al., 2017a).
Persea humilus (silk bay) is another species for which the taxonomy is confused. Some would regard silk bay as a species (Nelson, 1994), while others regard this taxon as a variety of redbay (Persea borbonia var. humilus) (Wunderlin, 1998). Silk bay is a small evergreen tree that is native to the scrub forests of central Florida. Laurel wilt is currently affecting silk bay in forests, and its susceptibility to the disease has been confirmed through pathogenicity tests (Hughes et al., 2012).
Lindera melissifolia (pondberry) is a small, deciduous, clonal shrub that is extremely rare and listed as an endangered species in the USA. Pathogenicity tests have determined that pondberry is highly susceptible to laurel wilt, but the disease has only been observed once in this species under natural conditions (Fraedrich et al., 2011). Because of its small stem diameter, pondberry is not readily attacked by X. glabratus. However, because of the clonal nature of this species, when infections occur, the disease can spread rapidly through rhizomes and kill multiple ramets within a population (Best and Fraedrich, 2018).
Lindera benzoin (spicebush) is a common small, deciduous shrub species that is found in the southeastern USA, and in pathogenicity tests it proved to be highly susceptible to laurel wilt (Fraedrich et al., 2008). The disease has been documented only once naturally in spicebush (Fraedrich et al., 2016), and because of the small diameter of spicebush, it is not readily attacked by X. glabratus. Therefore, the disease does not appear to be a major threat to this species.
Litsea aestivalis (pondspice) is a relatively large (0.5-3 m tall) deciduous, multi-branched shrub that occurs in the southeastern USA. The species is rare and is listed as threatened. Pondspice is highly susceptible to laurel wilt (Fraedrich et al., 2011), but due to the small size of this species, it is not readily attacked by X. glabratus. Laurel wilt has been observed in pondspice in Georgia and South Carolina (Fraedrich et al., 2011) and Florida (Hughes et al., 2011).
Licaria trianda (pepperleaf sweetwood) is a rare, evergreen tree native to the lower, southeastern portion of Florida. The species is considered to be endangered. A pathogenicity study determined that pepperleaf sweetwood was susceptible to disease caused by R. lauricola. Leaves of infected seedlings developed chlorosis and abscised, and a brown discolouration was noted in the xylem of stems. However, seedlings did not die from the disease (Ploetz and Konkol, 2013).
Ocotea coriacea (lancewood) is a small, evergreen tree that is found at scattered locations in central to south Florida and elsewhere in Central America and the Caribbean. Saplings inoculated with R. lauricola develop discolouration in the xylem and occasionally dieback of the branches but saplings do not die (S Fraedrich, US Forest Service, Georgia, USA, unpublished data).
Umbellularia californica (California laurel) is a large evergreen tree species native to southwestern Oregon, and the Coastal Ranges and Sierra Nevada of California. In laboratory pathogenicity tests, R. lauricola -inoculated plants developed sapwood discolouration and branch dieback but plants did not die from wilt (Fraedrich, 2008). A subsequent study also found that California laurel was an excellent brood host for X. glabratus (Mayfield et al., 2013).
Laurus nobilus (bay laurel) is an evergreen tree or large shrub species that is native to the Mediterranean regions of Europe, Asia and Africa. The taxonomy of Laurus nobilis and a similar species, L. azorica, which is found in Madeira and the Canary Islands, is confused and in need of review (Arroyo-García et al., 2001). Laurus nobilus was introduced into the USA, where it has been cultivated as a culinary herb and valued as a landscape ornamental species. Laurel wilt has been observed in a landscape plant in Florida and susceptibility of the bay laurel to the disease was subsequently confirmed in pathogenicity tests (Hughes et al., 2014).
Persea indica (viñatigo) is an evergreen tree native to the maritime forests of the Canary Islands, Madeira and the Azores. Viñatigo has been used as an ornamental in Florida and California in the USA (Schuch et al., 1992), and the species has been shown to be susceptible to laurel wilt in field and laboratory experiments (Hughes et al., 2013).
Cinnamomum camphora (camphortree) is indigenous to China, Japan, Taiwan and other countries in eastern Asia. The species was introduced into the southeastern USA in the 1800s and was used for camphor production, but has escaped cultivation and is now naturalized in some forest types (Langeland et al., 2008). Camphortree appears to be highly resistant to laurel wilt. Reports of laurel wilt in camphortree are not known in Asia, and in the USA the disease rarely affects camphortree in areas where redbay populations have been decimated by laurel wilt. Dieback in camphortrees is occasionally observed in trees where laurel wilt is prevalent on redbay, and R. lauricola has been recovered from such trees (Smith et al., 2009;Fraedrich et al., 2015b). Single point inoculations of camphortree saplings with R. lauricola do not produce symptoms of laurel wilt or dieback under controlled conditions;however, multiple inoculations with R. lauricola have resulted in top dieback and mortality in saplings (Fraedrich et al., 2015b).
In addition, a study of the susceptibility of lauraceous species native to South East Asia, indicated that Cinnamomum osmophloeum, C. jensenianum, Machilus zuihoensis and M. thunbergii were also much more resistant to laurel wilt than species native to North America (Shih et al., 2018).