I have many Crab apples that have been grown from seed over the past 15
years. Some are now starting to thicken up and a couple are flowering.
Some of the best shaped ones have developed crown gall.

That's what I get for starting at the *end* of a thread. Are you sure
this is crown gall? I don't know the situation in Wales; in the US galls
on apple roots are often caused by root aphids. Crown gall isn't common,
and isn't easily spread in a bonsai-situation (you'll want to sterilize
your pruning tools after use, however!). If I had a tree with crown-gall,
I wouldn't worry about it, but if I lived next to a commercial orchard,
I'd call Cooperative Extension and get their opinion on whether I needed
to take precautions as a good neighbor.

Folks in the US should look for root aphids, though.

Nina

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