Garden Pests and Diseases: Lemon tree - I think my tree is sick / brown spots, 3 by Mooncow

Mooncow wrote:
I planted a lemon seed 10 years ago and I got a nice little lemon tree out of it. Its more of an emotional thing for me:) I really like the little tree and its more than 1 meter long already.

So recently I noticed that the tree is rather weak and slumpy so I examined it more to see what could be the problem. And I noticed small brownish spots on the tree and on the leaves. Its seemingly some kind of bug egg or something but I'm not sure because I cant spot any actual bugs around the tree which could be eating it leeching his energy away.

Mostly the stock of the tree is having this issue and even there only the green part. The parts of the log which have some treebark too are intact and no brown spots are shown. The leaves are rather intact - only smaller dots are showing there and more rarely.

What treatment should I do? What can be this brown thing and is it dangerous to my tree? How can I avoid this happening in the future?

Thanks for every helpful reply.