Thank you for attaching the images to your inquiry.
Willow leaf galls are caused by the willow leaf gall sawfly, Pontania pacifica. Leaf galls, including these, are considered to be insignificant to the plant’s health. Thus, no treatment is suggested, even if they are very numerous. If the tree is of a reasonable size, and the galls are few, you could remove and destroy the affected leaves.
No sprays are recommended, or available, for willow leaf galls.
See “Willow gall sawflies’ at http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/willowgallsaw.html .And, here’s an interesting page that shows the 6mm long sawfly responsible for willow leaf galls, also several other tiny insects which help limit the gall makers - http://nathistoc.bio.uci.edu/hymenopt/Pontania%20pacifica.htm .