Pearleaf Blister Mite (Eriophyes pyri)
Blister mite are a microscopic pest that attack pears trees and occasionally apple trees.
The damage is done when the mites suck the sap from the underside of the new leaves causing the formation of blisters.
If the mite infestation is severe enough, the fruit can also suffer from dwarfing, malformation and depressed russet spots.
Mite eggs are laid with in the buds and in the blisters of the leaf for protection. Control the pests by removing effected branches and/or spraying with lime sulphur during dormancy to green tip stage.
A hot summer will reduce an infestation but it is likely some mites will still be active.