Blisters on oak leaves

There are two causes of blister-like galls on oak leaves; gall mites or a fungus, each with somewhat different treatment requirements. In both cases, typically the upper leaf surface is convex and the lower concave, although inversions are possible.Thankfully, there is a key feature to discern the two; mite galls have a velvet-like hairy underside, while the fungus lacks this and may have a grey colouration. In both cases the ‘blistered’ areas may die off.