Those are burls and can be very common with willows. Burls are a type of growth where the wood grain forms in a deformed manner and typically show as a rounded, knobby growth on the trunk or limbs. Causes are varied but they are not generally know to cause damage to the tree they appear on. Large burls tend to be very sought after by woodworkers for the stunning patterns they exhibit.

Just an FYI but willows are known to be prone to a whole slew of disease and insect problems as well as weak wood that can cause limb drop or splitting. The decline you are seeing could be a reflection of one or more of those problem as well as just a sign of old age.

thank you I was also wondering if I should be spraying it with some fungicide or just leave it

Just leave it :-)

If the tree has been getting subjected to drought conditions in later years the solution is to water it.

if you are really concerned about the tree .. you should have an onsite inspection by a certified arborist .. especially if it overhangs anything important ... peace of mind should be worth a few bucks ....

just add your city or state to the link

ken

https://duckduckgo.com/?q=isa+certified+arborist&t=ffnt&ia=web