So-called Walnut Petiole Galls produced by the Black Walnut Petiole Mite, Aceria caulis (family Eriophyidae), may be found on black walnut trees (Juglans nigra) in Ohio. The eriophyid gall mite is specific to black walnut and occurs throughout its native range.
Despite being commonly called “petiole galls,” the unusual growths may arise anywhere on the structural support members of the compound leaf. This includes the petiole, rachis, petiolule, and leaflet midvein.
The galls are covered in densely compacted hairs that resemble the erineum patches produced by other eriophyid mites. As the galls "mature," they transisiton in color from pinkish-white to pink to deep red and finally to dark brown.
The galls commonly cause parts of the compound leaf to become distorted and malformed. However, they seldom disrupt vascular flow. The affected food-producing leaflets remain functional even though they may be pointed in an array of directions.
The bizarre appearance of the gall growths, coupled with twisted leaves and leaflets, can cause tree owners to fear for the health of their trees. However, the eriophyid galls do not appear to cause significant harm to the overall health of their walnut hosts.
There are online recommendations to remove and destroy the galls on small trees to reduce the mite population. However, this could mean removing a considerable number of food-contributing leaves, making the treatment more damaging than the condition. Besides, as with many plant gall-makers, populations of this eriophyid appear to rise and fall dramatically from year-to-year. I seldom see heavy galling two years in a row on the same black walnut tree.
