Crusty Growth on Apple Tree

[BALLARDKREBS]

Hi,
This is my first post. I have espaliered apple trees that are 10 years old. One of them has developed crusty and large knobs on the branches. The leaf growth on this tree is much less than previous years. There are attached photos of the knobs.
Any ideas as to what they are and what to do about it?
Thanks,
Barbara

[fishpond]

Are you pruning correctly, when was the last time you fed it?

[yummersetter]

I think it could be woolly aphid / american blight - caused by white fluffy aphids.

Here's a link to the RHS page with more information Woolly aphid/RHS Gardening