There's nothing worse than that sinking feeling in the pit of your stomach when you catch a whiff of rotting roots or spot mushy stems or powdery deposits on your plants! Controlling diseases and pathogens like pythium and mildew is all part of growing plants and its always better to prevent these issues before they occur where possible.
There are a couple of approaches to to root disease control. You can innoculate root zones with friendly microbes and bacteria which will form something of a shield colony around your roots and prevent pythium from taking hold. Alternatively, you can run a sterile set-up, incorporating products like silver Bullet Roots into your nutrient regime to keep a completely sterile root zone.
In terms of leaf diseases and mildew etc, your first port of call should be addressing your environment to take away the conditions needed for a problem to occur. These include excessive humidity and poor air movement. Even if you get these right, you can still encounter problems - this is when you need to turn to defensive products such as incorporating Silicon into your feeds to strengthen your plant, or to foliar treatments to destroy the disease before too much damage is done.