Funny bumps on pear and plum leaves are caused by an Eriophyid mite called Pear Leaf Blister Mites. The injury may start as a reddish growth but as it matures it will turn dark brown. Heavily infested leaves may distort and fall off. A cool spring allows more injury than normal. This is only a cosmetic problem and once you see the problem it is too late to do much.