Could you do the same with the tree roots and put them in a bucket of strong
dye and wait to see which tree turned blue? If the dye isn't strong enough,
maybe a bucket of glyphosphate weedkiller so the problem tree and no other
would die? It seems sensible and must be cheaper than getting DNA testing
done?


Glyphosate isn't taken up by roots as far as I understand. It degrades in soil
too. You have to apply it to the green leaves alone to have any chance of
killing off any plant. If you could prevent escape into the drains/watercourse
applying sodium chlorate or something similar to thr tree roots might be the
answer. Even if it just caused enough damage to identify the tree that was
causing you problems it might be useful.

Dave.