"cliff_the_gardener"

As Michael says - wooly aphid.

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Removing them can be done with methalated spirits and a stiff brush if
you can get too them. If its on a big tree, a stong jet of water may
help to distrub them.

Snip

Thanks each. As it happens nearly all the "colonies" of them were on twigs
which were removed in pruning but now, any others are due for a dowsing in
meths.

Regards

Geoff