agastache black stems and dying

Pete.8

I grew about 50 agastache from seed this year and most are doing well, but in one patch of about 12 so far 5 have died.
All plants around them are fine, they are fed and watered - any ideas?

Another weekend where we can't post pics...

So Photos here-

http://imgur.com/a/eXwKI

Thanks

nutcutlet

I have agastaches in some very poor, dry soil doing fine. I wonder if they don't respond well to food and water.

Pete.8

Thanks nut. I grow them successfully every year in similar conditions, but there's something going on with this patch and it looks as if one-by-one they are getting black stems and dying for no apparent reason. Other patches of 10ish in the same border are fine

I know I've got Verticillium Wilt in another part of the garden and wonder if it's spreading and that maybe the cause. I certainly hope not!