yes Ree , I think you are right . I seem to remember that some of the old timers used the leaves to make a foul smelling liquid that would kill anything it touched , including humans, I think it stops your heart very effectively. I have heard about the thing where some people pop a bit of stalk or leaf in with the cabbages against club root, but have never tried it myself. If anyone does try the leaf water bit , go very careful ...never leave it where children or animals can get at it . It will KILL them. Like boiled tobacco , it will kill any if not all insects and bugs very well , just go very careful. I am not sure how good it would be at fending off clubroot though as clubroot likes acidic conditions, hence the reason for adding some lime to help fend it off . Raise the ph level and it will take much longer to take hold , by which time the cabbage has grown large enough to cope. However adding rhubarb leaf water would be adding Oxalic acid ( the stuff that kills you ) which would be adding to acidity... not sure about using the stalk though ?
cheers
Russ