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| 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.
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| 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