A teeny tiny little mite is responsible for those unsightly brown masses left behind over the winter on your ash tree!
These are ash flower galls. The mites overwinter under the flower buds, then begin feeding in the spring. The feeding then stimulates plant growth under the flower, thereby creating a gall. The gall hardens and turns brown later in the season, then hangs on for the winter.
Don't worry - the galls are purely an aesthetic issue and your tree won't be harmed. No need to control these mites!