What Causes Root Rot?

As a new orchid grower, one of the first pieces of cultural information we are given is that we should make sure to let the potting medium dry out between waterings, as the plants will otherwise get root rot. Some will even include the explanation that the constant moisture favors the growth of fungus, and that is what attacks and rots the roots.

Unfortunately, that is simply untrue.

If the mere presence of water was the cause of root rot, then there would be no possibility of growing orchids in hydroponic-, semi-hydroponic-, or water culture, all of which are quite viable techniques, or of wild plants surviving during the many months some experience in monsoon seasons, which is often when they thrive. If we take a different approach and blame it on fungi, then how would we expect orchids to grow in the wild, in natural environments that are no doubt overrun with all sorts of fungi, bacteria, and the like?

Instead, we should blame it on suffocation and poisoning, and consider what happens when we water a potted plant.

When we water, some (most) of the liquid simply runs through the pot, some of it is absorbed by the potting medium components, while more of it is held by surface tension in between the medium particles. Referring to the articles on Air Management and Particle Packing, we understand that the smaller the spaces between the particles, the more easily that so-called "bridging water" is maintained, allowing it to more completely cut off the air flow pathways.

If the air flow to the root system is stifled long enough, the gas exchange is compromised, and the roots die due to a combination of suffocation from the lack of oxygen, and poisoning by its own waste gases.

Once the root tissue dies, the natural resistance to pathogens is eliminated, and the roots will rot.

If you think about it, that mechanism is likely the misinterpreted basis for the "let the medium dry out" myth: as the plant absorbs moisture, and more of the liquid evaporates, those bridging water droplets go away, opening up the air flow pathways and allowing the root system to "breathe" again.

Rather than suffocating our plants at each watering, the better approach is to prevent the extensive occurrence of bridging water in our potting media, and keeping the spaces larger is the key:

Avoid mixing particles sizes. The smaller pieces fill the gaps between the larger ones, reducing the sizes of the void space.
Do not pack the potting medium too tightly. That is particularly important with sphagnum moss.
Repot into fresh medium on a regular, timely basis. As organic medium components age and decompose, they break down into smaller and smaller pieces, creating a dense and suffocating root environment.