Oooh! You grow Pineapples?

http://www.simplyhydro.com/root_rot.htm That's one answer that talks about preventing the rot.

Wikipedia talks about Hydrogen Peroxide being used, but what I've managed to pick up from other articles from Google (the majority about growing your own Marijuana crop) is that more oxygen is needed for the root - the root rot, (or Pythium? You'll know more than I), occurs in anaerobic conditions because the roots can't get enough oxygen --> for example, cold places.

Hope that helps.