Rad-Plague

The Rad-Plague was something the FL 400 Series D computer claimed was making the entire surface of Uthe 3 uninhabitable. Because most Uthians believed the computer implicitly, they never explored the surface to verify the claim. By producing a Uthanian scatterbud, the Fourth Doctor was able to disprove the computer's assertion. (PROSE: Sweet Flower of Uthe)

Behind the scenes
The story never says exactly what Rad-Plague is.