Hi:

Edgeworthia is such a rare landscape plant that I cannot find specific disease information on it. What I do know is that it can be a bit tender in this area. Although this winter was not especially cold, there was an extended period of very cool temperatures in January which did set back or even damage some semi-hardy plants. It's conceivable that the top growth may have been affected by this. If it is one stem that is affected, scrape the base of the shoot to see if the cambial layer is green or black, and if there is some point at which the green turns to brown. This might just be the point at which the top growth was winter-killed, or it might indicate the point at which the stem was affected by some sort of fungal or bacterial pathogen. If you do this and want to forward some photos of the damage to me, I might be able to draw a bit more in the way of conclusions as to the reason for the dieback. Email those to <personal data hidden>. 

Hi Neil,

Thanks so much for your response! In my case Winter damage isn't a likely culprit but I have talked to some other gardeners with older Edgeworthias who have also experienced this die-back and because the new suckers have healthy growth I'm taking that as a hopeful sign that I'm not dealing with a soil born disease. I'm going to cut out the main trunk and keep my fingers crossed that the new growth will remain healthy.

Thanks again!
Bria