Trees affected by leaf gall mites elicit strong reactions from novice gardeners. The leaves look diseased, sick and unsightly. We have one walnut tree that is showing some classic walnut leaf gall mite (Aceria erinoea) damage. The mites are tiny – 0.1 mm in size – and lurk overwinter in unopened buds, doing their damage before the leaf is properly developed. Unless the infestation is huge it will not cause the tree much strain, and is in any case very difficult to get rid of entirely. We’re not treating or spraying – just monitoring the blisters and reassuring visitors that it’s not contagious!