I am thinking that it is Oystershell scale. Does anyone know how to banish this stuff?

The silvery gray substance on the bark is lichen, which is completely harmless. But it appears the tree does have scale (second photo) - can't see close enough to determine what type but the treatment for any scale species will be similar. You can Google "oystershell scale" for various treatment approaches but a well established scale infestation is very difficult to control. And by appearances, it has already done a lot of damage to your tree.

It's your option. Remove and replace or try to get the infestation under control. Personally, I'd lean towards the first.

Oystershell scale - how to control.

That's got to be the worst infestation of scale I have ever seen!

Normally I would remove the bulk of the scale with a cloth then go after the crevice dwellers with alcohol on Q-tips, but of course given the size of this tree that is absurd.

The only viable option IMO would be to call in a pro who has access to the really top level chemicals and douse the hell out of it. However, looking at the current damage, even if this infestation was to be 100% controlled, the future growth would be... let's just say "uneven" for a very long time and the aesthetic value of the tree questionable.

If this is a highly sentimental tree and you have deep pockets, then go for it. Or even prune it back far enough that you can reach and treat all of it without standing on a ladder, and wait ten years for it to recover some sort of pleasing shape. You'd have to get out there very frequently and rub off all you can and treat chemically as well but IMO it could be done.

Otherwise it may be time to pull the plug on it. I hate to kill something that wants to live, and the good growth on this tree tells me it is trying it's best. It all depends on your tolerance for ICU level care and a poor appearance for quite some time.

Hi Guys

I am in total agreement with the above 2 Comments

I know very little about Plant diseases as of yet

I wonder whether this Tree produces Seeds.If so,a selection of Seedlings could possible be reared ?

I'm sure it would be Possible to Graft a Few Healthy Scions from the Tree.You could have half a Dozen Trees or more here from the Mother Tree.Perhaps the Story of this Ailing Tree could have a Happy & Romantic Ending.

A professional Local Nursery to you i'm sure could offer some sound advice about both the above suggestions

I am in agreement with Gardengal that the removal of the Tree would be the best Option here.The Tree has unfortunately suffered substantial damage and hasn't got the best Branch Configuration IMO.

If you do have a sentimental attachment to the Tree you will have to try your hardest to save it.

Healthy Scions could be Grafted in the Meantime and will need to be Carefully selected and checked

I wish you all the Luck

Stephen,UK