Laurel wilt is the consequence of an invasive species, the redbay ambrosia beetle (RAB) (Xyleborus glabratus) introduced into the United States in untreated wooden packaging material. It was first detected in diseased redbay trees (Persea borbonia) in Savannah, GA, in 2002 (Mayfield and Thomas, 2006). The beetle, originally from Asia, was found to be the vector of the disease in redbay trees and was thus named RAB (Fraedrich et al., 2008; Hanula et al., 2008; Mayfield and Thomas, 2006). The term ambrosia refers to an ecological relationship that these beetles share with fungal partners. These partners have coevolved to become dependent on one another for survival and dispersal. Ambrosia beetles belong to the subfamily Scolytinae and are some of the most common and the most devastating pests to plants known to date (Ploetz et al., 2013). Although its original introduction was through the RAB, it is important to note that in avocado, multiple beetle vectors have demonstrated the ability either as a contaminant or a new vector to inoculate trees (Carrillo et al., 2014).
Ambrosia beetles typically harbor fungal spores within specialized sacs known as mycangia, which excrete fungal spores as they bore into host trees, thus inoculating the tree with the fungus. The beetle then actively cultivates or farms the fungal gardens within excavated galleries as a food source for itself and its developing larvae (Batra, 1967; Ploetz et al., 2013). In laurel wilt disease, this particular fungal partner was identified as a lethal pathogen R. lauricola (Fraedrich et al., 2008; Harrington et al., 2008). The name, laurel wilt disease, was coined when the fungal spores of R. lauricola were shown to infect and lead to wilt symptoms in many species of laurel trees (Harrington et al., 2008), including the commercially important avocado (Ploetz et al., 2017a). This is a systemic disease incited by the clogging of the xylem vessels through the tree’s production of tylose-, phenolic-, pectin-, and lipid-containing gums or gels as it tries to defend itself from the systemic infection (Inch et al., 2012), a general response that can be produced because of several host insults. The result of blocked xylem vessels is compromised water and nutrient flow during evapotranspiration, visible wilting of the leaves, tissue necrosis, and, ultimately, the death of the tree. The presence of the fungus can be indicated visually through black/brown-stained sapwood, caused by the host response (Fraedrich et al., 2008; Inch and Ploetz, 2012; Inch et al., 2012).
Laurel wilt disease is highly aggressive and it has been demonstrated that only a few spores (as low as 100 conidia) or a single beetle-boring event can be sufficient to elicit the systemic disease in avocado. More than 300 million wild laurels have been lost and presently laurel wilt disease is rapidly spreading through the South Florida avocado groves, a $54-million/year industry (Evans et al., 2010), with the cost of replacement of trees rising to $400 million (Evans et al., 2010). This economic and ecological disaster will be even greater if this disease infects the larger avocado production areas such as California and Mexico.
Presently, the fungicide propiconazole (TILT®; Syngenta Crop Protection, Wilmington, DE) that suppresses the growth of the fungus but does not kill it is used to protect trees from infection. This formulation can provide 11–12 months of protection depending on the method of administration into the trees (Ploetz et al., 2017a, 2017b, 2017c). However, to prophylactically treat all trees comes with a high cost that many small farmers simply cannot afford. Studies geared toward early detection of laurel wilt disease using aerial imagine and remote sensing with drones have demonstrated promise in distinguishing between laurel wilt–affected trees from healthy trees as well as other stressors (de Castro et al., 2015a, 2015b). To aid in the early detection and control of this disease, scent-discriminating canines have been trained on laurel wilt–affected avocado wood to enable earlier detection and a more focused hot spot treatment that provides a cost-effective treatment plan for farmers.
Presently, canines are extensively used in law enforcement and forensics in the location of missing people, explosives, drugs, weapons, and ammunition (Furton and Myers, 2001). Furthermore, canines have demonstrated the ability to detect invasive species of spotted knapweed [Centaurea stoebe (Goodwin et al., 2010)], brown tree snake [Boiga irregularis (Savidge et al., 2011)], desert tortoise [Gopherus agassizii (Cablk and Heaton, 2006; Nussear et al., 2008)], and various cancers (Godfrey, 2014; McCulloch et al., 2006; Moser and McCulloch, 2010). They have even been used to detect bed bugs (Cimex lectularius), termites (Reticulitermes sp., Coptotermes sp., Cryptotermes sp., and Inscisitermes sp.), and mold (Aspergillus sp., Penicillium sp., and Stachybotrys sp.). The highly sensitive canine olfactory system, roughly 10,000–100,000 times more sensitive than humans, is capable of detecting odor concentrations at 1–2 parts/trillion. This is attributed to the size of the organ, density of neurons, the number of functional receptors, and the physical anatomy of the olfactory system (Craven et al., 2009). This study represents the first of its kind, to the author’s knowledge, evaluating the use of canine odor discrimination in the early detection of this plant disease.
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