Exploring Black Walnut Trees and Petiole Galls

By Kathy Salisbury
Director, Ambler Arboretum of Temple University

Those of you have participated in one of my walks and talks or attended any program about native trees (especially our early March More than Maples: Tapping for Sap - which was the last face-to-face public program we had - feels like eons ago doesn't it?) or have just happened to walk past trees with me, you likely know I have a passion for Black Walnut Trees. Juglans nigra, Black Walnut, are native trees to our eastern deciduous forest ecosystem. 

Many people know then their bad reputation of not letting other plants grow around them. While this is not the topic of this note about a curiosity, I will just say here that the fact that black walnuts grow in a plant community in our ecosystem proves they will let plants grow around them.

Anyway as I was investigating the branches of a black walnut closely recently I noticed a weird but decidedly cheerful pink growth on the petioles (stems) of the leaves. I have never seen anything like this. Perhaps you have noticed it too if you have dared be so bold to embrace the Black Walnut in all its edible nut-producing beauty and allow it to live around you. 

The pink formations are actually galls, which I wrote about in the last edition of Curiouser and Curiouser. These particular galls are Petiole Galls. These particular galls are caused by a type of mite - Eriophyid mite (there are many different types of these mites and others are responsible for galls in other plant species including Rose Rosette disease in roses). 

First discovered and named in 2015, these mites and the resulting pink galls are relatively new. Besides the pink fuzzy gall, the petioles twist around on themselves. The galls form when the mites feed on the petioles and the galls act as shelter for the growing mite colony.

Sadly, nut production declines in black walnuts infested with Petiole Gall. No control is necessary.