Nelson,
Thanks for capturing the perfect picture.
I see good green (leaves [-or where they used to be]) at one end,
and roots on the other,
with a bad (rotten) bark-break somewhere in between.
That is EXACLY what I meant.
Once a good-doing twig; it suddenly goes kaput...
The only "living" tissue of a twig is from just under the bark-out,
and once that rots, the root/leaf (plumbing) connection in broken.
As to what causes it, my prime-suspect is overwatering,
(not sure?) possibly combined with a less draining medium,
lack or air circulation and even not enough light?
(all encourage mold).
Any nasty "fungus gnats" larvea present may also cause damage
(for mold to settle-in).
[Very notorius to invade a GH/inside-home environment]
In my case, it does this not happen very often, but often enough for me
to notice. Unless, the twig is a "precious" one, now I do not even
bother to salvage it (not worth my effort).
Pete,
No, in my case, I did notice any obvious mold signs on the surface
(e.g., that tell-tale white fluffy stuff). By any chance,
did you use some potting meduim containing any not-so-ready compost?
[fungus gnats do love that stuff].