Arboriculture & Urban Forestry 43(6): November 2017 UNDERSTANDING THE ROLE OF INSECTS The DED fungus is not transmitted as an air- borne disease but rather is dependent on an insect vector. Extensive reviews of the elm bark beetle as they apply to DED have been pub- lished (Webber 2000; Santini and Faccoli 2014), so a brief treatment will suffice. Several species of bark beetles belonging to the genus Scolytus are the main vectors (Santini and Faccoli 2014), although Hylurgopinus rufipes can also transmit the disease (Bernier et al. 2015). The beetle vec- tors of DED carry fungal spores on the surface of their bodies. Given the lack of species speci- ficity, it is understandable that research on the vectors of elm disease is limited, and the focus is instead placed on understanding the fungus and DED tolerance in elms. It is known that the DED fungus causes elms to release four volatile terpenes that attract elm beetles. This may in- crease the probability of movement to adjacent elms (McLeod et al. 2005). There is also a species of mite (Tarsonemus crassus) sometimes found on elm bark beetles. They carry Ophiostoma spores within sporothecae (pockets adapted for fungal transmission), which can increase the spore load of the beetle (Moser et al. 2010). Stressed elms are more likely to become infected and succumb to DED, which by infer- ence means they are more attractive to beetles. Therefore, studies are warranted to determine if other insect pests of elms are problematic and increase the probability of susceptibility to DED. Repeated defoliation can encourage wood- boring insects (Miller 2000). Extensive studies indicate that the genus Ulmus has a plethora of insects that act as defoliators, miners, etc. (e.g., Miller 2000; Potter and Redmond 2013), and new vector species are still being discovered (Jacobi et al. 2013). Unique vectors have been recognized on elms that were introduced in the western U.S. (Lee et al. 2011). Conversely, species of elms have been evaluated for their insect resistance (e.g., Young and Hall 1986; Bosu et al. 2007; Condra et al. 2010; Potter and Redmond 2013). While few generalizations can be made, insect sus- ceptibility within an elm species is correlated to geographic region and interactions between specific insects and different elm species. From 225 a genetic standpoint, elm leaf beetle (Pyrrhalta luteola) appears to have a preference for hybrids with Ulmus pumila in their lineage (Miller 2000). UNDERSTANDING THE DED FUNGI While tree breeders bred new elms, pathologists and mycologists were trying to elucidate the biol- ogy of the DED fungus. The life cycle of fungi is brief, making genetic analysis much quicker than with trees. Since the fungus is the killer, under- standing its mode of action and genetic composi- tion is the beginning of developing a strategic plan to halt the disease. A detailed review on DED fungi has been published (Bernier et al. 2015); therefore, a brief overview and a short update will suffice. With DED, three fungal species are known to cause the disease. All are in the genus Ophiostoma, a dimorphic ascomycete with a mycelium/yeast tran- sition controlled by nutrition (Kulkarni and Nick- erson 1981). The less aggressive Ophiostoma ulmi dominated when DED first arose (Brasier 1991). By 1940, this species was largely replaced by the more aggressive O. novo-ulmi (Brasier 1991). Now, two subspecies of O. novo-ulmi are documented (O. novo-ulmi and O. novo-ulmi americana) (Brasier and Kirk 2001). Interestingly, a third species, O. himal-ulmi, has been identified as a naturally occur- ring endophyte on elms native to the Himalayas, where the elms are largely asymptomatic. When European elms are inoculated with this third spe- cies, it is pathogenic (Brasier and Mehrotra 1995). Ophiostoma novo-ulmi isolates have been col- lected from various parts of Europe and genetically analyzed. There is now evidence of hybridization between the American and Eurasian subspecies of O. novo-ulmi, and it has been shown that a recom- bination between two genes of the two subspecies of O. novo-ulmi can occur where the subspecies overlap in range (Konrad et al. 2002). Gene trans- fer is not limited within the genus. Geosmithia fungal species are almost always found in conjunc- tion with Ophiostoma, sharing the same vectors and habitat for a significant part of their life cycle (Pepori et al. 2015). Geosmithia has been shown to have the cerato-ulmin gene associated with Ophiostoma in over 50% of the 70 strains tested, but the gene is not active in this genus (Bettini et al. 2014). That being said, the movement of this gene between fungal genera is worth consider- ©2017 International Society of Arboriculture
November 2017
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