Theodor Otto Diener (born February 28, 1921) is an American plant pathologist and was the first in the field to describe viroids.
Diener was born in 1921 in Zurich, Switzerland. He came to the United States in 1949. In 1971 he showed that the causative agent of Potato spindle tuber disease was the potato spindle tuber viroid (PSTV). PSTV is a 130,000 dalton viroid. Viroids are very little single-stranded circular RNA that have no coat protein, and which do not code for any protein. Diener received the National Medal of Science in 1987 and the Wolf Prize in Agriculture in 1987. In 1999, Diener received Distinguished University Professor status from University of Maryland.