I got this mail:
>>> Mr Pall,
> I recently read your blog post on root rot and over watering. I would like for you to clarify something. Did you mean it is impossible to over water a well draining soil mixture?
> Then, what causes root rot?
Root rot can be two things. A rare fungus can infect the roots and kill them at least partially. They die and rot. This is a rare thing and has NOTHING to do with over watering. The fungus kills anyway, whether over watered or not, but it can be much more aggressive when the roots are all wet. Bonsai practitioners should NOT worry about this. It is very rare.
So what is the famous root rot that trees die off when over watered? Well, trees (apart from the 'real' root rot) don't die of root rot! And root rot is not necessarily a consequence of over watering with bad soil. It can also be under watering or many other things. It is simply this: For some reason roots die and then in a moist environment they will rot. When the bonsai enthusiast then finds that something is very wrong with the tree and takes it out of the pot he finds rotted roots. and He 'knows' that this caused the death. The truth is that rotted rots are a symptom, like fever. In the old days the would say of early settlers 'and they died of fever'. well, this was simply an acknowledgment that they had no clue what they had really, but they had fever anyway.
bottom line: TREES DO NOT DIE OF ROOT ROT! Trees die and then the roots rot. The gardener has to find out what was the cause of death when he sees the symptoms. you have to find the cause of fever.
When do trees die from over watering? When one uses an old-fashioned bad soil then there is not enough drainage. Water cannot flow off easily. Water will stay in the root ball and not keep it moist but wet. As a consequence the entry of new air with oxygen is blocked and roots suffocate. Roots must have oxygen in the air otherwise they will suffocate and die. Thus over watering in combination with bad soil can well kill trees and when they are dead they show root rot as symptom. Under watering can kill roots by simply drying out. When you then later on water again the dead roots will rot. it is the same symptom, but a radically different cause.
Modern substrates are very well draining and aerating. It would be difficult to over water. One can pour water into a bonsai with modern substrate for several minutes and all the superfluous water will flow out of the drainage holes and there will be free access of new air and thus roots will get oxygen to breath. If you insist you can over water still, of course. Just pour water in for hours. But who would do that?
Root rot is a bonsai myth.