Why is there bright yellow dust on my tree's leaves?

Sounds like rust - there are sprays to treat it, but as it's a tree, you might not be able to reach it properly.
Check for insect infestation to see if you can find anything causing the holes.
You could try using a combined fungicide/pesticide, but make sure the label doesn't say you shouldn't use on Salix.

Thank you Bamboo. Can't see any insect/pests and the holes seem to be where the "rust" is. If it wasn't a problem it looks rather pretty as it is so vivid yellow. I can reach as it is in a very large pot.