The rust species that grows specifically on Salix, especially Salix caprea and it's weeping Kilmarnock variety is quite a bright orange-red when releasing its spores. By the time you see this, its far too late to treat for this year and the horse has effectively bolted.

It does have a debilitating effect upon the tree though most people leave it, or don't bother until too late. You could try spraying next year with a suitable fungicide for rust but one thing I would do; Remove all fallen leaves from the base of the tree and any you find nearby. Once all leaves have fallen (naturally) and have been removed as best or as can be found from the ground, mulch around the base of the tree with composted bark to form a layer over any spores lying on the surface. This will stop rain splash sending spores back up onto the new foliage next year.