Nishiki Willows are a fantastic plant. They are much more appealing in the tree form than a ground variety. I have several of these and they do well in my zone 5 climate. They should be sheared severely every season to keep their growth curtailed and to provide the snow white growth that you may enjoy over the summer.
On shearing these trees, they may be cut back to the large woody branches where after about 3 weeks to a month, many new growth tips appear on these woody ends and the tree actually thickens significantly.
These plants thrive on lots of water and I mean water and not just dampness. They will grow 4-6 ft per year with ample water hence the shearing mentioned above.
There is nothing like them for their white growth for most of the season. I tend to keep them in the 8-10ft range in height.

Good Luck

I have had this problem on my white ash for three years running. Here is what the Kansas State University extension service says about it:
Recommendations: As with most galls, the Ash Flower Gall is unsightly but does not harm the
health of the tree. The mites are also difficult to control because they are able enter the flower bud
before it is visibly open. Therefore, control measures are not recommended.
hortster

I was hoping everyone was going to tell me the Gall would be detrimental to my tree....so I can replace it with a Salix. LOL. Not that I don't like the Ash, but Salix has so much more to offer.

My Ash tree, because of it's positioning, has more growth on one side than the other, so from the street it looks completely lopsided. Add to that those unsightly galls on it. Ugh.