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What is Ash Dieback Disease

We have recently been  surveying and delaing with the safety issues in relation to Ash Dieback Disease and thought we would provide some basic information. Hope you find this helpful.

Importance of Ash (Fraxinus excelsior).

Ash is a native tree of the UK, associated with some 955 species of flora & fauna, 45 of which are wholly specific to ash. Typically, this is a lowland woodland species at its northern and western distribution range, making up approximately 12% of all broadleaved woodland across the country. They have the potential for prolific natural regeneration in the right conditions. Initially they are a shade bearing species up to approximately 7 years in age, but then becoming more light demanding.

This species rarely suffers squirrel damage but is prone to the bacterial canker (Pseudomonas syringae ssp savastanoi pv. fraxini) particularly prevalent on poor sites and may fork due to damage caused by the Ash budmoth  (Prays fraxinella) and/or frost related damage to buds especially on east facing stands. The timber is not durable but is tough with good bending properties once seasoned correctly. Ash is a good firewood which can be burned green or seasoned.


Ash dieback disease (Hymenoscyphus fraxineus)

The condition issues caused by this disease can accelerate in a short period of time, resulting in extensive crown dieback and tree mortality.  The condition issues and symptoms associated with Ash dieback disease are explained further :-

Whilst Ash dieback disease (Hymenoscyphus fraxineus) is discussed here, poor health and/or structural issues maybe a symptom of other causal factors, either in conjunction with or as a stand-alone pathogen such as Honey fungus (Armillaria mellea), drought, poor growing conditions, Inonotus hispidus and Perenniporia fraxinea etc.

Ash dieback is a fungal pathogen present throughout the UK and leads to crown dieback and can cause lesions on the stem, particularly in humid conditions (Refer to Figure 1, below). Trees will vary in their response with some suffering serious crown dieback and can succumb to this alone or to a secondary pathogen, typically Honey fungus (Armillaria mellea). Alternatively, they may undergo periods of decline (sometimes rapid) and recovery.  The Health Stages are defined as 1-4 (Refer to Figure 2, bekow). A minor percentage may have genetic and/or site induced resistance, and it is these trees that are potentially important in retaining this species in the UK through natural regeneration and for seed collection for research, should this tolerance be inherited.

The disease is generally more prevalent in woodland type settings, where there is a high proportion of the host species with favourable humid conditions favouring the fungi’s life cycle.  Climate and site conditions play a large part in terms of the tree’s vitality with those under stress more likely to succumb.  With the associated leaf loss and crown dieback, the production of the stronger late wood decreases and over several years of infection this may lead to mechanically weaker stem, limbs and branches, with an increased risk of breakage.


Fig 1. Symptoms

Fig 2. Health Stages.

Source: The Tree Council: Ash Dieback : An Action Plan Toolkit. February 2019.

This information was correct at the time of publishing and is for general guidence only.

Please contact us if you have concerns regarding the condition of your ash trees and we can provide a quote for felling / tree surgery work, or arrange for one of our qualified arboricultural consultants to undertake a tree condition inspection.

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