Abstract
Dutch elm disease (DED) is a vascular wilt disease caused by the pathogens Ophiostoma ulmi and Ophiostoma novo-ulmi with multiple ecological phases including pathogenic (xylem), saprotrophic (bark) and vector (beetle flight and beetle feeding wound) phases. Due to the two DED pandemics during the twentieth century the use of elms in landscape and forest restoration has declined significantly. However new initiatives for elm breeding and restoration are now underway in Europe and North America. Here we discuss complexities in the DED ‘system’ that can lead to unintended consequences during elm breeding and some of the wider options for obtaining durability or ‘field resistance’ in released material, including (1) the phenotypic plasticity of disease levels in resistant cultivars infected by O. novo-ulmi; (2) shortcomings in test methods when selecting for resistance; (3) the implications of rapid evolutionary changes in current O. novo-ulmi populations for the choice of pathogen inoculum when screening; (4) the possibility of using active resistance to the pathogen in the beetle feeding wound, and low attractiveness of elm cultivars to feeding beetles, in addition to resistance in the xylem; (5) the risk that genes from susceptible and exotic elms be introgressed into resistant cultivars; (6) risks posed by unintentional changes in the host microbiome; and (7) the biosecurity risks posed by resistant elm deployment. In addition, attention needs to be paid to the disease pressures within which resistant elms will be released. In the future, biotechnology may further enhance our understanding of the various resistance processes in elms and our potential to deploy trees with highly durable resistance in elm restoration. Hopefully the different elm resistance processes will prove to be largely under durable, additive, multigenic control. Elm breeding programmes cannot afford to get into the host–pathogen arms races that characterise some agricultural host–pathogen systems.
Highlights
Species within the genus Ulmus found in Europe and North America were formerly considered keystone species, but nowadays this status is severely degraded due to the global impacts of two fungal pathogens in the genus Ophiostoma: the causal agents of Dutch elm disease (DED)
The first DED pandemic emerged in Europe in the first decades of the twentieth century and was caused by the introduction of Ophiostoma ulmi, which is moderately aggressive to European elms but highly aggressive to American elm (Ulmus americana) (Gibbs et al 1975)
The importance of monitoring pathogen populations is emphasised by the arrival of O. novo-ulmi in the 1940s, which has resulted in the steady replacement and near extinction of O. ulmi across Europe and North America
Summary
Species within the genus Ulmus found in Europe and North America were formerly considered keystone species, but nowadays this status is severely degraded due to the global impacts of two fungal pathogens in the genus Ophiostoma: the causal agents of Dutch elm disease (DED). During more than 11 years of elm inoculations at the same site using the single elm genotype U. procera SR4 (= U. minor), and the same set of O. novo-ulmi genotypes, the ranking of the pathogen genotypes remained the same but disease levels fluctuated from moderately resistant to highly susceptible, the main environmental influences being temperature and light intensity (Fig. 3; Sutherland et al 1997). Besides plasticity in DED resistance, other factors not always considered in elm breeding programs could result in elm restoration failure These include intolerance of selected cultivars to frost, flooding, wind and drought, and the relative attractiveness/susceptibility of selected cultivars to pests and pathogens such as scolytids, Xanthogaleruca luteola, Cossus cossus, Stegophora ulmea, Nectria canker and elm yellows (Smalley and Guries 1993; Pecori et al 2017). In the following sub-sections our understanding of the factors that influence resistance at different phases of the disease is reviewed
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