Actually it's a water mold, related to brown algae. Common on box and on other plants. One kind causes sudden oak death. Think I've read this one may transported by rain, otherwise infestations are conventionally associated with avoidable conditions of wetness, such as puddling, impeded drainage and splashing water. Once a plant is infested you may not be able to clean it up, but there has been at least one product available that is supposed to contain an existing infestation.

There is one product called Alietta by Bayer Chemicals. Hopefully the other sample will be clear (the roots were white not brown). I think Monday will be spent inspecting roots of all the boxwoods to see which are having problems and which are currently clear. Then decide how to proceed. I get to chat with the irrigation guy Tuesday and when that goes in we might well be in for another round of drowned roots from leaky connections!

Subdue was being used on rhododendrons in a park near here some years ago.

The way I read your post, the problem didn't occur until after the plants were moved, perhaps triggered by the stress. My experience with phytophthora has been such that by the time dieback showed up in the top, the roots were brown. I had this happen in other species, not boxwood.
When you say 'we have some going red' this sounds like what I think of as Boxwood Blight or Decline, that section will also be stunted. Read about it in the link. We have lost some beautiful old shrubs due to the blight. By the time you cut out the dead part, the shape of the plant is ruined.
I assume that we are talking about English Box?
Sam

Here is a link that might be useful: Boxwood Fact Sheet

Sam
Some of the redness I think is just trauma - the roots are white and the occurrence of redness is not increasing at a rapid rate. This is only occurring on transplanted boxwoods, the English kind.
The phytopthora has been confirmed by the university lab, so is definately present on some. Right now I am checking the roots of all the tranplanted ones and trimming back to 12" or so any that seem to be declining - some are still trying to put out new growth at the base. The predominant feeling is that the rootball was too small on some of the larger specimens. Many of the smaller ones are still fine.
Some, of course, just gave up the ghost, went that bleached look, flopped open and died. There is a temptation to curse, cajole and generally yell at the darn things when they start that decline business, because by the time you see the start, it is usually too late to remediate.
Thank you for the link.

I would only cut off top growth that is dead or dying (or has a poor shape, after they have perked up) because top growth supports root growth (and vice versa). But maybe that IS what you are doing.