A 'pear' of blisters!
Many thanks for your recent enquiry regarding Pear Leaf Mite. This is not a pest that has caused disruption in our own Harrod Horticultural Kitchen Garden but I do have some information which should certainly prove helpful.
Let's learn a little about the pest first. Adult Pear Leaf Blister Mites (eriophyes pyri) feed on young pear leaves and cause the distinctive pink and yellow surface blisters to appear. Mite colonies then develop in the tissues of blistered leaves and the tree begins to suffer; symptoms include severely infested leaves dying and young shoots being growth checked. The blisters on infected leaves turn brown and black prior to the leaf dying and the crafty mites move out of leaves before they fall in autumn and overwinter in dormant buds. Although the symptoms can look serious, the damage to the tree is negligible (apart from the details above) and the best policy of control is to pick off infected leaves as they are seen. This will break the life cycle of the mite and prevent serious infestations developing. Any form of chemical/organic spray control is unlikely to have any effect as the mites are safely tucked away in the leaf tissues, so it’s a case of being vigilant and removing leaves by hand. You may get some benefit from garlic sprays as they build up natural resistance in plants but they will certainly not solve the problem on this occasion.
Hopefully this brief resume will prove helpful; although the pear leaf blister mite is not one of the most potent pests in the garden, it's still useful for grow your own enthusiasts - be it fruit, veg or both - to know the effects of these creatures and how much time, effort and expense it is worth devoting to controlling them.
The best of luck with your pears this year!