Arboriculture & Urban Forestry 43(6): November 2017 it one of the most economically troubling pathogens in both North America and Europe (Lee et al. 2004). The pathogen inhabits the phloem sieve elements in stems and collapses them, causing the tree to starve (USDA Forest Service 2012). The disease is not as pandemic as DED, being more prevalent in the eastern half of the U.S. (Sinclair 2000). Elm yellows was a significant problem in the 1990s. Aſterward, reports of outbreaks declined. The incidence of in- fection appears to be on the rise, perhaps because the urban American elm population is increasing aſter the introduction of DED-tolerant cultivars (Peduto- Hand et al. 2014). Unlike the case with DED, leaves on elms infected with elm yellows do not wilt and turn brown but rather suddenly turn yel- low. To date, there is no practical treatment or cure. The current taxonomic status of the elm yel- lows phytoplasma calls for the name ‘Candidatus Phytoplasma ulmi’ (Jović et al. 2011). At least four indigenous elm species can be infected (Sinclair 2000). Some Eurasian elms appear to be tolerant or resistant (Sinclair 2000; USDA Forest Service 2012). Several experimentally infected Eurasian elms, already in the trade, did show some signs of elm yellows infection (Sinclair et al. 2000). As with DED, the pathogen can move from tree to tree via root graſt or requires insect vectors, not beetles as with DED, but leafhoppers and spittle- bugs with piercing or sucking mouthparts (Peduto- Hand et al. 2014). Vectors of the phytoplasma include the leafhoppers, Scaphoideus luteolus and Allygus atomarius, and the spittlebug (Philaenus spumarius) (Sinclair 2000). More recently, another leafhopper genus (Latalus sp.) and another spit- tlebug species (Lepyronia quadrangularis) have been added to the list of vectors (Rosa et al. 2014). Control of elm yellows consists of removing and destroying infected trees to reduce regional patho- gen load. Mittempergher (2000) states that while screening for DED tolerance, testing for susceptibil- ity to elm yellows is prudent. Genetic sequencing of the pathogen indicates that there are many sub- groups within the genus that can cause the disease (Jović et al. 2011). Detecting the pathogen generally involves a DNA analysis, such as restriction frag- ment length polymorphism and polymerase chain reaction (Sinclair et al. 2000; Herath et al. 2010). Detection is useful for testing the resistance of differ- ent elm genotypes but not as a preventative strategy. 227 GENETIC ENGINEERING: A NEW ROUTE TO ELM IMPROVEMENT Conventional methods (i.e., hybridization, screen- ing) have significantly improved DED tolerance in American elm. Yet, the life cycle of elms is not conducive to multigenerational breeding programs, and it has taken many decades to select Ameri- can elms that tolerate DED enough to make prudent use of them sensible. With advances in molecular biology, genetic and biochemi- cal research has increased, as has knowledge of DED, its pathogen, its vector, and elm biology. One option to combat diseases is the generation of genetically modified organisms (GMOs). With crop plants, genetic engineering for fungal resis- tance has been demonstrated in numerous species (reviewed by Ceasar and Ignacimuthu 2012). In this regard, analysis of American chestnut (Cas- tanea dentata) research is informative to studies of American elm. According to Powell, “chestnut research has provided additional candidate genes that could be tested in elm in the future” (W.A. Powell, personal communication). American elms and American chestnuts were both devastated by an exotic fungus. In the case of chestnut, the blight is caused by the fungus Cryphonectria parasitica, which is a windborne pathogen that attacks the tree trunk by growing a network of mycelia that deposit oxalic acid. This eventually destroys bark and causes cankers that girdle the tree (Powell 2014). Both Ulmus americana and Castanea dentata have Asian relatives within their genus that show resistance to the respective disease, perhaps owing to their evo- lution with their native pathogen over countless years. Both American elm and American chestnut have undergone conventional breeding and genetic engineering. The chestnut can be bred to a blight- resistant Asian species, the Chinese chestnut (Casta- nea mollissima). The F1 hybrids are fertile and can be backcrossed repeatedly to American chestnut in an attempt to regain the American chestnut phenotype in addition to the resistance genes from Chinese chestnut. Third backcross progeny are being field tested (Pinchot et al. 2015). In contrast, it is nearly impossible to hybridize American elm with other elms. In the rare cases where a hybrid is reported, no successful backcrosses could be made (Bey 1990). The lack of a backcrossing strategy to Asian species is a great hindrance to American elm improvement. ©2017 International Society of Arboriculture
November 2017
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