Biology of Chalara ash dieback pathogen

Our research is exploring the biology of the fungus including the genetic recognition system known as vegetative compatibility (vc).

Research objectives

The initial objectives of this research are to:

  • Assess the extent of diversity in recently arrived populations of H. fraxineus by exploring the genetic recognition system of the fungus known as vegetative compatibility.
  • Compare the diversity of H. fraxineus and H. albidus.
  • Establish how long H. fraxineus may have been causing disease in ash in the UK prior to the first finding of the pathogen in 2012.

Results so far

Vc reaction between different individual of H FraxineusOur work has shown that the ash dieback pathogen, H. fraxineus, has a somatic self–nonself recognition or vegetative compatibility ‘vc’ system,that is readily expressed in culture. It is the equivalent of a tissue-rejection system in humans and enables some fungi to distinguish between self and non-self genotypes in culture and nature. For H. fraxineus, the vc reactions appears as gaps or pigmented lines when individuals are paired in culture . Similar reactions are seen in naturally infected ash leaves indicating this recognition process also takes place in nature. Almost every individual of H. fraxineus tested so far is a unique vc-type suggesting high genetic diversity even at local population levels. Similar work is now underway with H. albidus.

As we examine ash dieback sites across the country, there is accumulating evidence that the pathogen may have been present for several years before the first formal findings in 2012. This illustrates the difficulties of early detection when trying to intercept new and potentially invasive pathogens.


The ash dieback pathogen Hymenoscyphus fraxineus (synonym Hymenoscyphus pseudoalbidus) is also known by the common name of Chalara fraxinea. It is a recently arrived pathogen that has proved highly destructive elsewhere in Europe and now threatens our native ash (Fraxinus excelsior). Since its discovery in the UK in 2012, surveys have shown that ash dieback disease is now established in many parts of south east England. Apart from killing many ash trees it is likely that this pathogen will cause the extinction of its native cousin H. albidus which is harmless to European ash. As Chalara disease caused by Hymenoscyphus fraxineus invades Britain, the aim of this research is to gain a better understanding of the pathogen in terms of its behaviour, genetics and interaction with the native H. albidus.


The project started in April 2013 and is ongoing


Dr Joan Webber and Dr Steven Hendry

Funders and partners

This research project is funded by the Forestry Commission Advice and scientific support for tree health programme

It is undertaken in partnership with Forestry Commission England Plant Health.

Forestry Commission policy

This research underpins the evidence base for the delivery of healthy and resilient forests and wider ecosystems which is part of the Tree Health and Plant Biosecurity Action Plan