Yellow marks on kale

I have blue vates curled kale, red russian and nero di tosca, all growing really well but now they are all getting yellow sorts of bubbles on them.  I have been trying to cut the affected leaves straight away, but if I leave them they get a sort of white powder in the 'bubbles'.  I assume it is some sort of desease, does anyone have any suggestions please as I don't want it to spread any more.  

Thanks
Ali