I have two buxus which have suffered a lack of water due to irrigation failing in the summer and no time to hand water them. There is a definite orange tinge to the first 2 or 3 inches of top growth. I fear pruning at this time of year will kill them. Will a good fungicide at least kill the disease. Advice please? Would be much appreciated.
With the Buxus or Boxwood it is very common for the plant to discolor in winter, especially if they're not protected from the sun and wind.
I would bet you're experiencing drying and not rust. A picture would confirm that though.