Pearleaf Blister Mite can be a tricky problem as there are no chemicals available to gardeners for its control. The adults overwinter in the leaf buds and lay their eggs here in spring. The leaf blisters are caused by mites feeding on the developing leaves and these blisters are later colonised by mites until just before leaf fall in autumn, when they return to the buds to spend the winter months.
The only real control option that you have is to interrupt that life cycle by picking off any infected leaves as soon as they are seen. Make sure that you destroy them to prevent infection recurring.