Theodor O. Diener, a Swiss-born scientist whose investigation additional than half a century ago of shriveled, stunted potatoes yielded the discovery of the tiniest identified agent of infectious illness, a particle one particular-eightieth the size of a virus that he named the viroid, died March 28 at his house in Beltsville, Md. He was 102.

His son Michael Diener confirmed his death but did not cite a bring about.

Dr. Diener immigrated to the United States in 1949 and spent 3 decades as a plant pathologist at the Agricultural Study Service, the chief internal study agency of the U.S. Division of Agriculture.

President Ronald Reagan awarded him a National Medal of Science for his identification in 1971 of the viroid, an achievement that has been compared in its significance to the discovery of bacteria in the late 1600s and of viruses shortly just before the turn of the 20th century.

Considering that roughly the 1920s, farmers had identified of a confounding illness that threatened their potatoes, leaving them shrunken and malformed and decreasing the size of a crop by 50 % or additional. In the 1960s and 1970s, according to a report in Forbes magazine, as quite a few as half of the potato plants in some places of China and Ukraine had been sickened.

The situation had a name — potato spindle tuber illness — but its bring about proved vexingly elusive.

Functioning for years with colleagues at ARS, Dr. Diener was credited with identifying the infectious agent at the proverbial root of the challenge. Applying study solutions such as centrifugation, he determined that the bring about was not a virus, as other scientists had speculated, but rather a new, far smaller sized pathogen — the viroid.

A viroid functions in a manner related to that of a virus, invading a cell and producing it reproduce the viroid’s RNA. In contrast to a virus, a viroid has no protein coat. According to the ARS, the extended-standing consensus amongst scientists was that such “naked” pathogens had been unable to replicate, even with the help of an infected cell.

Most scientists also believed that a pathogen as minuscule as the one particular Dr. Diener found was incapable of invading an organism. But Dr. Diener’s study proved that a viroid — so little that it is barely visible even with an electron microscope — can certainly mount an productive attack.

Soon after identifying the viroid, Dr. Diener helped create a test to detect the one particular that causes potato spindle tuber illness. Viroids had been later discovered to bring about situations such as tomato chlorotic dwarf, apple scar skin, avocado sunblotch and chrysanthemum stunt. There are additional than 30 identified species of viroids in plant pathology.

Dr. Diener’s National Medal of Science, awarded in 1987, credited his discovery with making “new avenues of molecular study into some of the most really serious illnesses afflicting plants, animals, and humans.”

Theodor Otto Diener was born in Zurich, in the German-speaking component of Switzerland, on Feb. 28, 1921. His father was a postal employee, and his mother was an accountant. Even in his youth, the future scientist was drawn to plants and animals.

“As a boy, I often kept animals at house: turtles, salamanders, frogs, white mice, hamsters,” he when told an interviewer. “Whereas my parents exhibited a significant dose of tolerance to this, neighbors generally did not.”

His curiosity led him to ever tinier creatures, he stated, just after he saved adequate income to acquire a secondhand Leitz microscope.

Dr. Diener studied biology at the Swiss Federal Institute of Technologies, getting a doctoral degree in 1948.

Soon after immigrating to the United States, exactly where he became a naturalized U.S. citizen, he worked as a plant pathologist at Washington State University just before joining ARS in Beltsville in 1959. He collaborated on ARS study extended just after his official retirement in 1988.

He also carried out study and taught at the University of Maryland, exactly where he was a professor emeritus.

In addition to the National Medal of Science, Dr. Diener’s honors incorporated election to the National Academy of Sciences in 1977 and the American Academy of Arts and Sciences in 1978. In 1987, he received the Wolf Prize in agriculture, a $one hundred,000 award provided by the Wolf Foundation in Israel.

In addition to his prolific scientific papers, he wrote two books, “Viroids and Viroid Diseases” (1979) and a memoir, “Of Humans, Humanoids and Viroids” (2014).

Dr. Diener’s marriage to Shirley Baumann ended in divorce. His wife of 44 years, the former Sybil Fox, died in 2012.

Survivors involve 3 sons from his very first marriage, Theodore Diener of Los Angeles, Robert Diener of Urbana, Ill., and Michael Diener of Vienna, Va. 5 grandchildren and 3 terrific-grandchildren.

Like quite a few scientific advances, Dr. Diener’s discovery of the viroid was the effective outcome of quite a few earlier unsuccessful tries. Reflecting on his efforts to decode potato spindle tuber illness, he wryly told the New York Instances, “we went up the garden path quite a few instances.”