Hops shoots drying out

I'm currently in my second year of growing hops, with two Magnums and two Mt. Hood. My Magnum rhizomes were larger and stronger, and threw up tons of shoots this year. I trimmed back to about six per plant, and they grew pretty quickly to about six feet. Now they've stagnated for over a month and the tips of the shoots have dried up and turned brown, at least on the Magnums. The Mt. Hoods still seem to be okay. We have had a warm spell recently, but I noticed the browning tips before the really hot weather hit. Any idea what could be wrong? I'm assuming there won't be any more vertical growth from these shoots.

What is your water/feeding schedule?

firts guess id say a mineral deficiency, perhaps boron. pics are a remarcable way to get better help though.