Virology Division News
Arch Virol 145/11 (2000)
Letter to the editor
In his paper “The post-Loeffler era: contribution of German virologists” (in: “100 Years of
Virology,” C. H. Calisher and M. C. Horzinek, eds., Arch. Virol. Suppl. 15, p. 43–61, 1999),
R. Rott states: “Certainly, one of the most remarkable discoveries in plant virology in the
post-Schramm era was the simultaneous and independent finding by Theodor O. Diener in
the USA and Heinz L. Sänger in Giessen of ‘naked’ small RNA molecules as a new kind of
autonomously replicating subviral plant pathogens known today as viroids.”
This statement is demonstrably wrong. In a paper published in August 1971 (Virology
45: 411–428), I presented conclusive evidence that the agent of the potato spindle tuber
disease (now known as PSTVd) possesses unique properties that distinguish it from all
viruses, thus representing the prototype of a novel class of pathogen, for which I proposed
the term viroid.
Sänger, after having heard my presentation at the 1971 International Congress for
Virology and being cognizant of my 1971 Virology paper (op. cit.), simply applied the
methodology developed for PSTVd to another plant pathogen, the agent of the citrus
exocortis disease (then known as ExC).
That Sänger himself agrees with this chronology is evident from statements he made in
his very first (albeit unreviewed) publication on viroids (H. L. Sänger, Adv. Biosc. 8: 103–
116, 1972): “Therefore, I conclude that ExC is the second member of the new class of
minute infectious agents for which the term ‘viroids’ has been proposed .” And: “The
agent of the spindle tuber disease of potato (PSTV) was the first viroid discovered …” Ref.
 is my 1971 Virology paper cited above.
In conclusion, Sänger deserves credit for confirming the viroid concept with a second
plant pathogen, but certainly not, as Rott claims, for “simultaneously and independently”
discovering the viroid.
Prof. em. Th. O. Diener
Center for Agricultural Biotechnology
University of Maryland
College Park, MD 20742
* Editor’s note: Following a suggestion by Prof. M. C. Horzinek, co-editor of “100 Years of
Virology” (together with Prof. C. H. Calisher), the following discussion between Prof. Diener and
Prof. Rott is published here to make it accessible to a wider audience.