Ash dieback due to Hymenoscyphus fraxineus: what can be learnt from evolutionary ecology?

Abstract

The future existence of common ash (Fraxinus excelsior), an important tree species throughout temperate Europe, is threatened. An invasive fungal disease (ash dieback) has spread through much of the distribution area of common ash. The causal agent of the disease is Hymenoscyphus fraxineus, a necrotrophic ascomycete, most probably introduced from Asia in the early 1990s. Hymenoscyphus fraxineus infects ash trees and saplings through their leaves, from which it grows into the stem. The fungus was studied intensively in recent years but there is still a need to address the topic from an evolutionary perspective. In this overview, some key evolutionary aspects of ash dieback are discussed, from the Red Queen dynamics of host–pathogen interactions to the probable consequences for virulence evolution of multiple infections. The progression of ash dieback in Europe does not show spatial differences, but studies show variation in susceptibility within host populations, a probable consequence of genetic differences, thus providing material for evolution of disease resistance or tolerance. Breeding programmes need to maintain the genetic diversity of Fraxinus, to enable it to withstand further threats such as climate change and the emerald ash borer. Because H. fraxineus reproduces exclusively sexually, the pathogen is likely to overcome a narrow genetic resistance. The introduction of further strains of H. fraxineus to Europe and the movement of infected plant material should be avoided. This case study shows that the integration of evolutionary ecology considerations would benefit plant disease management and biosecurity in general.

Evaluating the impact of Hymenoscyphus fraxineus in Trentino (Alps, Northern Italy): first investigations

“…However, the presence of asymptomatic trees in the surveyed area seems to suggest the existence of resistance or tolerance to the disease (Queloz 2016, Landolt et al 2016. These plants could constitute the genetic reservoir for implementing the conservation of the species.…”

“…overcome restrictive genetic resistance if evolutionary processes favour fungal genotypes of higher virulence (Landolt et al 2016). The level of damage has drastically hindered the silvicultural perspectives of European ash in Trentino forests.…”

The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history

“…Currently, shoot infection of European ash is believed to result from pathogen hyphae growing into the stem via the leaf petiole before leaf shed (Gross et al 2012;Landolt et al 2016). Since the fruit bodies of H. fraxineus are abundantly formed on leaf veins and rarely on shoot tissues, leaf colonization is key factor to the invasiveness of this fungus, whereas colonization of shoot by the pathogen, even if a useful trait to score host susceptibility, can be considered a dead end in the life cycle of H. fraxineus.…”

“…Regarding potential pathways of introduction of H. fraxineus to North America arrival via airborne ascospores seems less plausible than introduction via import of infected but asymptomatic plants, seeds (Cleary et al 2013a) or wood (Husson et al 2012). Given the higher genetic variation (Cleary et al 2016;Gross et al 2014b) and presumably also higher virulence of East Asian strains (Gross and Sieber 2016), introductions from East Asia would be especially critical (Landolt et al 2016). Our results highly support measures taken to prevent unintended introduction of H. fraxineus via ash products to North America, a scenario that would likely result in species decline in one of the two main centers of diversity of genus Fraxinus.…”

Effect of fungal pathogens and climatic factors on production, biodiversity and health status of ash mountain forests

“…Nevertheless, high economic as well as ecological value of the species and of its timber challenges the reintroduction of this species in the forest stands (Goberville et al, 2016). Ash decline affects many other plant and animal species linked with it (Rackham et al, 2014;Landolt et al, 2016).…”