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Is ash dieback under control?

Image by FERA
European ash tree is an important component of British woodlands. It has been stayed popular and recommended for planting due to its economic and aesthetic value, also the fact that its resistance towards grey squirrels. In UK, it has been estimated that among all the 141000ha big woodlands (>0.5ha), 5.4% of their composition is ash trees. However, since its first discovery in Poland in 1992, the ash dieback disease, caused by fungus Chalara fraxinea, has spread over the European continent and devastated ash populations in certain areas. On 19.Sep, Rob Spence for Forestry Commission came to Bristol to talk about thecurrent stage of ash dieback control in England.

Chalara fraxinea is the asexual stage of Hymenoscyphus pseudoalbidus, and also the infectious stage. Ascospores are produced from fruiting bodies on the dead branches in the litter, and can be transmitted by wind to more than 10km. Ascospores are not durable, thus its infection window is limited to summer months. The spores tend to attack the young trees due to their lower resistance to the disease, cause crown necrosis and eventually death. In mature plants, the effect of the disease is less severe. However, the disease can seriously compromise the condition of mature trees, and make them succumb to other diseases.

Source: BBC website
Current distribution of the disease in England is largely constrained in tree nurseries, except for East Anglia, where a number of cases have been reported in the wild. The prevalence of the disease in the nurseries all over the country is thought to be due to the fact that seeds are germinated outside of UK, and then saplings and young trees are imported back into UK from the continent, which may already be infected. However, the large outbreak in East Anglia is more likely attributed to extreme weather conditions which bring spores from the continent.

The control effort in southwest is focusing on confining the disease. Unlike East Anglia, the cases of ash dieback in wild are still rare. The Forestry Commission has been conducting aerial surveys to spot early infections, also, two smartphone apps, Tree Alert and OPAL can be used to take photos of suspected infected trees and send to the experts for identification. As the staff of the Forestry Commission is very limited, it becomes very unrealistic for them to come to field for most cases.

It is also worth noting that around 1-2% of the natural population is resistant against the disease. Researches are going on in The Sainsbury’s Lab and John Innes Centre in Norwich, as well as some European institutes trying to identify the resistant genes and possible approaches to deter the spread of the fungus through biological approaches. On country level, a ban has been placed on ash import from outside of the country and transfer of living ash tissues within the country, though the timber transport are still allowed as they are regarded as low risk.

In my point of view, ash dieback is well controlled at this stage. Despite the eventual widespread is inevitable, but this kind of selection bottlenecks has happened widely in nature since the evolution starts. Although there is no reason to reduce our effort in protecting ash trees, as long as we keep the genetic diversity with the susceptible populations while introducing and expanding the resistant traits within the population, the disease will be controlled in macro-scale. 

This blog is written by Dan Lan, Biological Sciences, University of Bristol


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