The cause of a potato disease bedeviled scientists for decades before Dr. Diener figured out that an impossibly small pathogen called a viroid was to blame.
Theodor O. Diener, a Swiss-born scientist whose investigation more than half a century ago of shriveled, stunted potatoes yielded the discovery of the tiniest known agent of infectious disease, died March 28 at his home in Beltsville, Md. He was 102.