Dead plants. What's wrong?

Hi folks,

Out of seven hop varieties planted this Spring two have died in the last three weeks, and one seems to be struggling. First the leaves turn yellow/brown, then an entire bine dries out within 3-4 days. Here are the pictures:

http://bergengarden.tumblr.com/image/58083040170
http://bergengarden.tumblr.com/image/60708698285

Varieties that died are Ultra and Liberty. Also, Mt. Hood is struggling. I have a downy mildew problem with my grapes that are growing nearby. But the underside of hops leaves doesn't have those white spots.

Any ideas? Thanks.

Are you fertilizing at all?

Not actively. I relied on humus/manure that I mixed into the soil back in Spring, during planting.

Yesterday I started spraying "fish and seaweed" fertilizer. Hoping this will turn the tide.

Turns out I overwatered the hops. Dug up a rhizome today:

http://bergengarden.tumblr.com/post/61252394056/mystery-solved-i-killed-the-hops-by-overwatering