Root rot stems from overwatering. It is not actually a disease or a problem, but a symptom of something that has already happened.

Despite what is commonly known, plants do require oxygen to metabolize their food the same way we do. For the top part of the plant, thats no problem. Photosynthesis creates more of the stuff than the plant uses, so that part is good. The roots, however, do not photosynthesize and must get oxygen from the surrounding environment, aka the soil.

When you keep the soil too wet, the roots literally start to drown, just like you or I would. As the roots die, the plant becomes incapable of getting water and nutrients to the top of the plant. After the roots die, they start to rot, hence you see root rot. It isn't root rot that kills the roots, its the dead roots that cause the rot.

The same also applies to giving them too much chemical fertilizer, you literally burn off the small feeder roots and you get the same basic symptoms as if you were overwatering (although generally on a much faster timetable).

Signs of overwatering are actually the same general signs as under-watering; drooping, curling, and/or yellowing leaves.

If you see your plants are having trouble, do *NOT* give them more water, as you may very well be making the situation worse.

As always, make sure your pots have sufficient drainage holes and that you are not using a moisture control potting soil, as those tend to stay far too wet for most plants to be happy in. Ideally you should be able to pour a large amount of water into your pot and see it pouring out the bottom a few seconds later. If you don't, you need to be extra careful to only water when the plants tell you they need a drink. Fixed schedules will likely kill them.

If you see the signs of overwatering and root rot, the first thing you should do is remove the plant from the pot its currently in. Odds are it will be soppy wet in there, and thats bad. Immediately rinse the roots off to get rid of the old soil, and replant it in a better draining mix with lots of perlite.

If you catch it early, you will at most just set the plant back a few weeks. If you catch it late, it may not recover.

If you catch it late and its a rare/valuable plant that you absolutely want to save, that is when you should look into foliar sprays. They are not ideal, but when the roots are so damaged that the plant is dying, they can be a stop gap measure long enough for the plant to heal itself and put out new roots.

Pythium, a pathogen that comes with root-rot, can be controlled with hydrogen peroxide.