Thank you for the excellent pictures you sent with your question. I believe your ash trees are suffering from Ash Flower Galls, which are caused by an infestation of Eriophyid mites. These mites are too small to be seen without the aid of a hand lens. This condition is considered cosmetic in nature, and the mites do not harm the tree's health or growth.

Additional information can be found here:
https://hort.uwex.edu/articles/ash-flower-gall/

We also have some general information on mites and galls in this Extension Factsheet:
http://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/

Regards,