I think we are trying to take a few, small pieces of information and apply them a bit too broadly.
Yes, cinnamon powder applied liberally to the roots of a plant will desiccate them. That does not automatically mean that a chunk of cinnamon bark will do the same. If that was true, then
any cinnamon present in contact with the medium or roots would cause such desiccation, which it does not. You can treat a cut root tip with the powder, and it will branch above the cut and be fine.
The fungus killer in cinnamon is cinnamaldehyde, not simply its desiccating properties. You can extract the cinnamaldehyde with water, and the extremely dilute - and certainly
not desiccating - solution will kill fungus.
However, even though I doubt that a few pieces of cinnamon bark in the medium will hurt anything (but I'd experiment on a plant or two before trying it on a large scale), I'm not sure it would be any great benefit, either. Here's where I'm coming from on this:
I think that most orchid roots do not die from exposure to pathogens, whether they are fungal, bacterial, or whatever. I believe the plants naturally combat such things - otherwise there would be no orchids in the wild, as most of their habitats are rampant with such things. Instead, I pin most orchid root death on suffocation. (Reread Samantha's first post in this thread, and you can see where that might be the case here.) Once the roots suffocate, they die, and THEN fungi and bacteria can go to work on them.
Based upon that, the culprit in potting medium is not the presence of fungi, but the suffocating effect of too dense media, exacerbated by being waterlogged. (Read "
What Causes Root Rot?".) Adding a few pieces of cinnamon bark isn't going to do anything to help that directly.
However, I suppose there is the possibility that the presence of cinnamon might reduce the population of the microorganisms that break down bark, slowing the rate at which is decomposes and densifies. Hmmmmmm....
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