What's killing my Aconitum (Monkshood)?

Hi all,

I planted 3 Monkshood at the back of my shady border and all 3 seemed to be growing well. Two days ago I noticed that one of them had started to badly wilt and turn yellow. It's now shrunk quite a lot! All the other plants around it seem to be doing well (including the 2 other Monkshood). See attached pics.

Any ideas?

 

Vine weeval? Didn't think of that - I'll investigate, cheers. Doubt it could be overwatering, as it gets no more water than anything else.

I was worried it might have been verticillium wilt (which sounds like a nightmare!)