I wish to start by saying that this is by no means a thoroughly extensive literature review. The below is merely a primer, of sorts, for how we currently sit in the bid to breed elms (Ulmus spp.) resistant to Dutch elm disease (Ophiostoma novo-ulmi). Furthermore, this is written from the angle of someone who lives in England, so if there are some Anglo-centric comments then you know why! So, without further ado…
Following on from the mass mortality of elms across the UK’s landscape in recent decades (and also an earlier spate of the disease during the first half of the last century), there has been a huge desire to breed resistant stock, in an attempt to have elms grace the landscape once more. However, such a pursuit has been very slow in terms of substantial progress, and it is only in more recent years that more significant developments have begun to manifest in breeding resistance against Ophiostoma novo-ulmi. In fact, not all of these advancements have been in the UK – research across Europe and the US has really helped shape the way forward for the elm, given the disease was responsible for the loss of elms across both continents (Lamb, 1979; Venturas et al., 2014).
Because resistance to Ophiostoma novo-ulmi (DED) is considered to be largely polygenic (qualitative) – in the sense that resistance does not follow a major gene pattern, but instead by the effective production of phenolic compounds and signalling metabolites (compartmentalisation processes), xylem morphology (length, width), and so on – breeding resistant stock is automatically very difficult (Aoun et al., 2009; Ďurkovič et al., 2014; Martín et al., 2013a; Martín et al., 2013b). Therefore, for resistance to be identified, understanding what genetic markers to look for is necessary, and research is ongoing in this regard (Perdiguero et al., 2015). However, because the main manner in which polygenic resistance is cultivated is through vegetative propagation, there is huge risk of a future strain of Ophiostoma novo-ulmi, or even another pathogen, creating the same problem that arose over the last few decades with regards to elm mortality (Martín et al., 2013b). A small genetic pool across many individuals is not to be desired, and can spell disaster very quickly – as was the case for Ulmus minor, which had a history of vegetative propagation across the English landscape and suffered great losses as a result of DED.
With regards to identifying resistant individuals therefore, research has indeed been completed via the growing of cuttings from different mother trees for a variety of species (including Ulmus glabra, Ulmus laevis, Ulmus minor, and Ulmus pumila), and then inoculating the cuttings with Ophiostoma novo-ulmi (Solla et al., 2005). Results have shown that some cuttings of particular species will display complete resistance to the pathogen, though many cuttings of other species will not. The reason behind evident resistance is largely considered to be down to xylem width and length, with individuals that possess longer and broader vessels showing greater susceptibility to the pathogen (Pouzoulet et al., 2014). This may not exclusively be in the wood structure either, as leaf xylem structure also impacts upon resistance – ‘Dodoens’ is marked as a potentially important cultivar, in this regard (Ďurkovič et al., 2014).
To advance this displayed resilience of some cuttings, hybridisation projects have been undertaken that may even see resistant European elm individuals crossed with resistant Asian ones – this broadens the genetic resources an individual will have access to (Brunet et al., 2013; Santini et al., 2012; Solla et al., 2014). If such resultant individuals display resistance in the laboratory, then they will be propagated vegetatively and then be subjected to further testing in a more naturalised environment (Santini et al., 2010). Such a means of testing for resistance has produced some clones that may potentially be used in an ornamental setting or forestry setting, including ‘Ademuz’, ‘Majadahonda’, and ‘Morfeo’ (Martín et al., 2015). It is, at this point, important to recognise that the resistance of different clones may in fact vary across regions of the world, and that cultivars will also differ in their response to the pathogen – some will initially display low resistance but soon recover and exhibit few signs the following year, and some perhaps vice versa (Buiteveld et al., 2015). Furthermore, current research has only really focussed on trailing resistance in young specimens. Little evidence is available to show how these new cultivars will fare in the longer term (such as over many decades).
Concerningly, research by Hodgetts et al. (2015) in the UK found that freshly-imported ‘Morfeo’ clones were host to Candidatus phytoplasma ulmi, which is a pathogen that is controlled in the UK and therefore requires infected stock to be destroyed under a Plant Health Notice. Despite this, many older and pre-existing clones were not found to be host to the pathogen. Therefore, risk may also exist with regards to the movement of cultivars, which evidently may harbour exotic pathogens that may threaten the UK’s tree species. On a similar note, Ulmus americana ‘Princeton’ (a DED-resistant cultivar) has been discontinued by some nurseries (including Barcham) because of its high susceptibility to Candidatus phytoplasma ulmi.
Very recent research has also identified specific genetic markers in Ulmus minor that may suggest resistance (Perdiguero et al., 2015). Such identification, the authors allege, may aid significantly with the quest in finding disease-resistant cultivars, and such markers may also be transferable across species. At the same time, the genomics of the pathogen Ophiostoma novo-ulmi, now fully mapped, is paving the way for innovative research that seeks to identify specific markers that identify pathogenicity (Bernier et al., 2015). Research into the pathogen itself may therefore yield beneficial results in understanding how to breed for resistance, though only time will tell in this regard.
Even in spite of the cultivation of many individuals that display levels of resistance to DED, the fact that vegetative propagation is the main means of continuing to provide resistant elms means there is huge risk of elm populations lacking genetic diversity. Such populations are fragile, and can readily be wiped-out by a pathogen in a very quick period of time. Of course, looking to re-introduce elm to the landscape through means of cultivation is a noble pursuit, particularly when man was the main cause of the second DED outbreak in the UK, though it is perhaps naive to think that a similar thing could not happen again – and by planting clones, a similar mass-mortality event has a much higher likelihood of occurring. Hybridisation of elms is therefore potentially a way forward, in place of cloning. However, then we run the risk of a loss of true native progeny, and crossing species that would never otherwise have the ability to cross may be ethically obstructive for some.
Pessimism aside, understanding what drives resistance is important, and DED has triggered a great deal of research into finding resistant elms. The benefits of such research – very importantly – does not stop with elms, as research techniques can be replicated for other areas of disease research. Technological advancements and continued investigatory work into DED will therefore continue to yield good results, though potentially with greater magnitude. Judging by current research, finding truly resistant cultivars is indeed a possibility, and planting a large mix of them when they do arise may be the best way forward.
References
Aoun, M., Rioux, D., Simard, M., & Bernier, L. (2009) Fungal colonization and host defense reactions in Ulmus americana callus cultures inoculated with Ophiostoma novo-ulmi. Phytopathology. 99 (6). p642-650.
Bernier, L., Aoun, M., Bouvet, G., Comeau, A., Dufour, J., Naruzawa, E., Nigg, M., & Plourde, K. (2015) Genomics of the Dutch elm disease pathosystem: are we there yet?. iForest-Biogeosciences and Forestry. 8 (2). p149-157.
Brunet, J., Zalapa, J.E., Pecori, F., & Santini, A. (2013) Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy. Biological Invasions. 15 (12). p2717-2730.
Buiteveld, J., van der Werf, B., & Hiemstra, J.A.. (2015) Comparison of commercial elm cultivars and promising unreleased Dutch clones for resistance to Ophiostoma novo-ulmi. iForest-Biogeosciences and Forestry. 8 (2). p158-164.
Ďurkovič, J., Čaňová, I., Lagaňa, R., Kučerová, V., Moravčík, M., Priwitzer, T., Urban, J., Dvořák, M., & Krajňáková, J. (2013) Leaf trait dissimilarities between Dutch elm hybrids with a contrasting tolerance to Dutch elm disease. Annals of Botany. 111 (2). p215-227.
Ďurkovič, J., Kačík, F., Olčák, D., Kučerová, V. , & Krajňáková, J. (2014) Host responses and metabolic profiles of wood components in Dutch elm hybrids with a contrasting tolerance to Dutch elm disease. Annals of Botany. 114 (1). p47-59.
Hodgetts, J., Flint, L., & Fox, A. (2015) First report of ‘Candidatus phytoplasma ulmi’ (16SrV-A) associated with Ulmus cultivar Morfeo (elm) in the United Kingdom. New Disease Reports. 32 (1). p26.
Lamb, R. (1979) World Without Trees: Dutch elm disease and other human errors. UK: Wildwood House.
Martín, J., Solla, A., Ruiz-Villar, M., & Gil, L. (2013) Vessel length and conductivity of Ulmus branches: ontogenetic changes and relation to resistance to Dutch elm disease. Trees. 27 (5). p1239-1248.
Martín, J., Solla, A., Venturas, M., Collada, C., Domínguez, J., Miranda, E., Fuentes, P., Burón, M., Iglesias, S., & Gil, L. (2015) Seven Ulmus minor clones tolerant to Ophiostoma novo-ulmi registered as forest reproductive material in Spain. iForest-Biogeosciences and Forestry. 8 (2). p172-180.
Martín, J., Witzell, J., Blumenstein, K., Rozpedowska, E., Helander, M., Sieber, T., & Gil, L. (2013b) Resistance to Dutch elm disease reduces presence of xylem endophytic fungi in elms (Ulmus spp.). PLoS One. 8 (2). e56987.
Perdiguero, P., Venturas, M., Cervera, M., Gil, L., & Collada, C. (2015) Massive sequencing of Ulmus minor’s transcriptome provides new molecular tools for a genus under the constant threat of Dutch elm disease. Frontiers in Plant Science. 6 (541). p1-12.
Pouzoulet, J., Pivovaroff, A., Santiago, L., & Rolshausen, P. (2014) Can vessel dimension explain tolerance toward fungal vascular wilt diseases in woody plants? Lessons from Dutch elm disease and esca disease in grapevine. Frontiers in Plant Science. 5 (253). p1-11.
Santini, A., Pecori, F., Pepori, A., & Brookes, A. (2012) ‘Morfeo’Elm: a new variety resistant to Dutch elm disease. Forest Pathology. 42 (2). p171-176.
Solla, A., Bohnens, J., Collin, E., Diamandis, S., Franke, A., Gil, L., Burón, M., Santini, A., Mittempergher, L., Pinon, J., & Broeck, A. (2005) Screening European elms for resistance to Ophiostoma novo-ulmi. Forest Science. 51 (2). p134-141.
Solla, A., López-Almansa, J., Martín, J., & Gil, L. (2014) Genetic variation and heritability estimates of Ulmus minor and Ulmus pumila hybrids for budburst, growth and tolerance to Ophiostoma novo-ulmi. iForest-Biogeosciences and Forestry. 8 (4). p422-430.
Santini, A., Pecori, F., Pepori, A., Ferrini, F., & Ghelardini, L. (2010) Genotype× environment interaction and growth stability of several elm clones resistant to Dutch elm disease. Forest Ecology and Management. 260 (6). p1017-1025.
Venturas, M., López, R., Martín, J., Gascó, A., & Gil, L. (2014) Heritability of Ulmus minor resistance to Dutch elm disease and its relationship to vessel size, but not to xylem vulnerability to drought. Plant Pathology. 63 (3). p500-509.
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