Sounds like scale insect problem to me. They are sap suckers and weaken a young or unhealthy tree badly. Heavy infestations can even cause problems on larger trees.

Not sure how you would treat a tree. I have only dealt with a small plant in a conservatory. I am sure someone else will be able to advice you further.

I just leave mine, some years are better than others, what with that, leafminer on the horse chestnuts and now a fungus on ash trees its a bit of a worry......

I hope this thanks is in the correct place............sincerely greatful

this is a spangle gall. To the victorians they looked like spangles /sequins. its a wound response to a sap sucker. it is not fatal to the tree and may not occur next year. sweep up fallen leaves and they should be disposed of.