Pathogen Profile #8

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

This latest newsletter is all about ash dieback, a disease which has become a significant concern due to its devastating impact on ash trees (Fraxinus spp.) across Europe.

Ash dieback is caused by the ascomycete fungus Hymenoscyphus fraxineus. Originating in Asia, disease damage was first detected in Poland in 1992. The pathogen then spread rapidly across Europe, causing severe declines in ash populations. By 2012, it had spread throughout the continent, affecting both common ash (Fraxinus excelsior) and narrow-leaved ash (Fraxinus angustifolia).

The pathogen spreads primarily through airborne ascospores, initially infecting the leaves of Fraxinus species. As the fungus moves through the leaves, it eventually spreads to the branches and trunks which causes necrosis and crown dieback. With the dead leaves now fallen to the ground, small, white (somewhat adorable) fruiting bodies emerge the following summer/autumn growing period. Ascospores are released, and the infection cycle begins again.

Ascospores have been shown to carry long distances, with one record of travel up to 100 km in France. Coupled with the wide distribution of common and narrow-leaved ash, this has resulted in the rapid spread of H. fraxineus in Europe. Human activities such as the movement of infected plant material also contribute to the spread of the pathogen, making it a significant threat to both native and cultivated ash populations.

Ash dieback poses a serious threat to economically valuable plants, impacting forestry and horticulture sectors. Ash trees are commonly used in urban landscaping, timber production, and as a source of firewood. Economic losses are also associated with tree removal and lost benefits, such as carbon sequestration. In the UK alone, ash dieback is estimated to cost £15 billion in tree removal and lost benefits.

Beyond its economic impact, ash dieback also threatens biodiversity. Ash trees play a crucial role in many ecosystems, providing habitat and food for a variety of wildlife. The decline of ash populations can lead to a reduction in biodiversity, altering ecosystems and affecting other plant and animal species dependent on these trees. A Swedish study identified 115 species at high risk of regional extinction due to ash declines.

Efforts are underway to manage and mitigate the impact of ash dieback. Researchers and government agencies are collaborating on strategies such as the identification of resistant ash tree varieties, enhanced monitoring techniques, and public education campaigns to prevent the spread of this disease. Biological control methods and fungicides are also being explored as potential management tools.

If you’re interested in reading more about ash dieback, I’ve summarised a few articles below. If you’re a subscriber, scroll down to access my online database which includes full summaries of these, and all other Robigalia featured articles.

Hymenoscyphus fraxineus articles

Hymenoscyphus fraxineus persistence in the ash litter

• The study investigates the persistence and decomposition of ash litter colonised by Hymenoscyphus fraxineus. The study found H. fraxineus can persist on rachises for several years, even under natural conditions, and continues to produce apothecia. This persistence creates a reservoir of inoculum that can reignite local outbreaks even after unfavourable weather conditions.

Traces of Hymenoscyphus fraxineus in Northeastern Europe Extend Further Back in History than Expected

• This study examined historical herbarium specimens of Fraxinus spp. to determine the presence and timeline of Hymenoscyphus fraxineus. The research determined the presence of H. fraxineus in Europe from herbarium specimens collected in Estonia in 1978 and 1992 suggesting H. fraxineus was present in Europe 14 years before the first recorded damages in Poland.

Evidence of a component Allee effect for an invasive pathogen: Hymenoscyphus fraxineus, the ash dieback agent

• The study investigates the component Allee effect (a positive association between absolute average individual fitness and population size) in H. fraxineus. The study determined the pathogen's spread is slower in areas with lower host densities, limiting disease development and provides evidence that the component Allee effect in H. fraxineus affects inoculum production, which in turn influences the pathogen's epidemiology and invasion dynamics.

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Until next week,

Alyssa