Mites – but none were found under the microscope, nor insects. However, Eriophyid mites are very small and feed in the growing point and on very young leaves. They will not be found on the expanded leaves and they are normally very hard to find even in the growing points. A miticide could be tried, although in a backyard this might not be desirable. The damage tends to be seasonal, so the problem may simply disappear as the plants get larger.

It was pointed out that there are also Tenuipalpid mites (not Eriophyid). Look for tiny active shiny mites.

There were other suggestions, mealybugs, virus and even Erwinia, a bacterium; however, mites were thought to be the most likely.