What causes this?

Broch

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Jan 18, 2009
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That's common on Ash. I've got one or two like that on 25 year old trees but others around them are fine. That's not Ash die back (which looks different) it's pseudomonas syringae, bacterial ash canker (though there may be other bacteria/fungal infections that cause similar scars and growths).

Advice for woodland which is managed for timber is to remove it but when the wood is managed for bio-diversity and ecology, as mine is, it's left because it provides habitat.
 
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Broch

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Jan 18, 2009
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Actually no; it can affect a number of trees including prunus varieties. Yes it is contagious but doesn't seem particularly aggressively so. It is not a notifiable disease either and there is no general advice to cut and burn. I have perfectly healthy ash all around infected ones and infected trees carry on maturing and live for quite some time. However, when badly affected they become structurally weakened so, if somewhere people walk, it is probably worth removing them.
 
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Broch

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Jan 18, 2009
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Mid Wales
Is this what causes the wonderful 'spalting' in the grain of the timber ?
Possibly but in my experience it is very close to the surface and when it has penetrated any depth into the tree the fibres are very brittle and unusable. Having said that I am keen now to go out there and slice some up and polish it :) - I'll let you know how I get on!
 
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