Zestar Issue

I was walking through the orchard the other day looking at the trees and clipping off any branches that are broken.  As I walked through the Zestar trees I noticed a tree that was dead from the very top of the leader to about a foot down. The leaves and any flowers and blossoms were dead. As I kept going through the trees I noticed more trees here and there with the same condition. I didn't get a good count but I'd say that I have at least 10 trees have the problem. It almost looks like fire blight without the ooze.   I contacted the ISU extension service and they think it is winter injury or something with the roots from the nursery.  I guess I'll call the nursery on Monday and see what they think.