Does anyone know what could be causing this weakness in the root of my ficus ginseng?
Welcome to the Empire, Ben.
It's rotting, this much we already know.
Could be fungus, could be excess moisture, could be an insect infestation.
Ficus can take a lot of abuse, so if you have to chop this root to save it, it will probably survive the trauma.
The Helpful Gardener says:
"Ginseng Ficus varieties; they are prone to a certain kind of root rot, that I call Ginseng rot. What happens is, for unknown reasons (although I believe it is related to letting the soil dry out), a fungus (that is, from my experience, harmful only to ginseng varieties, probably because almost all the plant consists of 'root') will invade the roots of the tree. It then decays all the vascular tissues on the inside of the root. The infection stops at the trunk or branches, but the plant is no longer able to obtain water, because the roots have been killed. The plant then, for lack of water, loses its leaves, and dies. Once it strikes a root, it is best to amputate it. One can tell if a root is infected due to a shriveled, wrinkled appearance, and a rather soft feel to it rather than a smooth, succulent look and normal firmness. However, if the fungus infects all the roots, it is usually too late. This fungi is, from my experience, little known because I have never seen it mentioned anywhere. Also, be sure to check out this site for some more information on growing ficuses indoors:
www.bonsaihunk.us/cultural.html