Looks like some kind of rust infection - if the tree is small enough, spray with a fungicidal treatment that includes rust on its list of diseases, and which can also be used safely on fruiting plants.

also could be the start of leaf blister mite.my pear tree started looking like little brown raised dots,then turned into black bits,fruit may become blistered aswell.pick the leaves off and burn, like bamboo says fungicidal sprays cover alot of the problems on pear trees.

Looking at a picture in Hessayon could be Magnesium shortage. Advises watering with a chelated fertilser. I used Epsom salts...magnesium sulphate on my Carpentaria when I identified same deficiency from a picture in RHS Garden mag. Had a rapid response and cheap from the chemist too. water in around tree. Mulch and feed as well. I don't think an essential element like magnesium can do harm if no deficiency. My Carpentaria is looking great now but I nearly lost it.