I have this brown stuff growing on all the leaves on my pear tree in my garden.
Does anyone know what this is? I think it might be some plant disease.
Does anyone have any advice on fixing this?
Aha, thanks for the picture, makes all the difference. Here is a link to Pear Rust Gall; please take a look at the information provided and see how it compares. There are other pages addressing the same issue on pears that may also help narrow it down. Are you in the UK?
The fix for this is to prevent spores from reaching the pear tree. The alternate host for the fungus is junipers, so keep an eye on local junipers to see if they show signs. If so, consult with your local experts what can be done. Also collect and burn infected leaves as soon as possible.