Melampsora caprearum is a fungal pathogen which causes galls on willows (Salix species). Also known as a rust fungus, it was first described by Felix von Thümen in 1879.
Melampsora caprearum distorts the blades and veins of willow leaves, causing irregular spots with yellow-orange uredinia (which produce a powdery mass of spores). The rust has been found on eared willow (Salix aurita), goat willow (S. caprea), grey willow (S. cinerea) and their hybrids.
Has been recorded from Belgium (photo), Finland, Great Britain (common) and Poland.