Blistered leaves and warty twigs are signs of eriophyid mites.
Eriophyid mites are a family of microscopic plant parasites that include blister mites, gall mites, bud mites, and rust mites. While you will probably never see them without a 20x hand lens, at 1/100” in length, they can be a translucent yellow, pink, white, or purple, with two pairs of legs up near the head.
Symptoms of eriophyid mite feeding include:
Blister mites commonly attack beech, boxelder, cottonwood, elm, maple, live oak, walnut, willow, poplar, roses, privet, and alder. Unfortunately, they also feed on tomatoes, peach, apples, pear, and grapes. I have also heard of them on plum trees in the Bay area. Eriophyid mites spread on the wind, so there isn’t much you can do to stop them from showing up in your landscape. Luckily, the damage they cause is mostly aesthetic and poses no real threat to otherwise healthy plants. That being said, who wants warty leaves or scabby twigs? Also, eriophyid mites may also carry viruses that can cause real damage.
Insecticidal soaps and horticultural oils can be used on heavy infestations, but simply removing affected leaves, buds, flowers and twigs is really all that’s needed. Sulphur and neem oil have also been shown to be effective. Before you work too hard at getting rid of these minor pests, you may want to keep in mind that the eriophyid mite serves as a replacement food for predatory spider mites and other beneficials, when their foods of choice are absent.
Isn’t it nice to know that some problems in the garden are not big enough to worry about?
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.