Emerging leaves of Fraxinus spp. in the spring are often seen to be abruptly divided into dozens of narrow, deformed leaflets, some of which may be extremely narrow. This phenomenon often affects one leaf of an opposing pair and not the other, or may even begin halfway along the length of an otherwise normal leaf. In other cases, even the petioles and twigs are affected, causing extremely dense clusters of abortive tissue.

This symptom is often attributed to Aceria fraxiniflora, which affects flowers on the same hosts. However, according to Amrine, Aceria fraxiniflora is distinguished from its European counterpart Aceria fraxinivora by the lack of leaf damage. It's possible this exclusion was mistaken, or that Aceria fraxinivora is responsible for these leaf symptoms (the mite has been found in North America, though its leaf galls in Europe don't resemble the symptom discussed here). Likely the cause is something entirely different and yet to be discovered--possibly fasciation induced by mutations within the tree itself. For the moment, many observations can be found in this observation set on iNaturalist.