Uncurl one and look carefully, perhaps with magnifying glass for pests that are sucking the juices out of the leaves, such as aphids. I would remove those leaves (too late for them to recover) and treat the good ones with an insecticidal soap. Aphids and spider mites can also be blasted off with a fine but strong spray of water, scale cannot, and may also be on the branches.

Most likely a species of leaf gall thrip