I have a thicket of Sumac that suddenly is really not doing well...a walnut tree was camouflaged until it recently popped up above the sumacs...could that be poisoning them?
Rhus typhina and R. glabra are supposed to be resistant to juglone. Perhaps root competition is a factor.
Evidently not, per lycopus' findings. Is the walnut large enough to be shading out the sumac grove? Are individual stems of the sumac succumbing to some sort of wilt?
As far as I'm concerned, sumac is a nice plant but not one to fuss over too much. Each individual stem is of short duration. What matters is that the clone itself stays alive, and most often, they do this by pushing new stems in new locations. IOWs, any individual stem may not last all that long, but new ones should be appearing regularly.
Not sure that helps at all, but that's what I can say from this distance.