If there is a sewage smell to the rotting roots and the soil, this is anaerobic decay caused by poor drainage and overwatering. Manure and rotting wood do not smell like sewage if oxygen is present. So the prime question would be whether that nasty sour smell is present below the soil surface.

Ordinary canker does not normally, if ever, spread wildly through the stems and roots. Roses are able to defend themselves against canker and wall it off. (A partial exception might occur during winter dormancy when mild, wet weather allows the fungus to be more active than the plant's immune system.)

It is possible that your roses have something like phytopthera, but I would look first at the soil conditions. 50% manure is way too much. The muck would fill all the spaces between the sand grains and, if regularly watered, remain saturated with water. That much manure might also create a toxic excess of salts, and particularly if you have been fertilizing.

Use either a standard American potting mix such as Miracle Gro, or a light sandy loam containing about 80% large particles (sand or other gritty material, bark fines, coir). There is good advice on the Container Forum.

Since coming to southern California I've seen a lot of dead roses. Most of my neighbor's roses look this way. I would attribute it to lack of proper care. The soil here can be totally dry and rock hard. Most people don't mulch or water correctly. Aerial watering runs off or if it is trapped it just sits there because there is no drainage and the rose can rot. You really have to build up the soil and get proper drainage and once you do you have to feed and water heavily especially when it stays hot and dry for long periods. And BTW, old woody canes with no basals are common here. It is a matter of time before roses like this die.