I lost a huge RoS several years ago. It was sudden and looked like yours. I sent soil and the roots to our extension agent and found out it hd cotton rot
Here is a little info, I hope it isn't
Cotton Root Rot
Cotton root rot is prevalent in alkaline soils like those found in Texas. In some areas the disease is called Texas root rot. Leaves and branches will wilt and die but remain attached to the plant. The disease is diagnosed by examining the roots of the affected plant at a 10X magnification to identify light brown strands or webs. The fungus invades the soil and the bush should be transplanted to a container using sterilized potting soil. Soil treatment must be done by a licensed applicator at one- to two-year intervals. Only plant specimens that are resistant to cotton root rot in affected soil.
Read more: Rose of Sharon Disease | Garden Guides
