INTRODUCTORY REMARKS TO THE SESSION “ETIOLOGY OF VIRUS AND VIROID DISEASES OF FRUIT TREES”

Virology starts as a plant discipline approximately one century ago as a consequence of studies aimed at identifying the etiological agent of tobacco mosaic disease.
There are relatively few similar situations in the history of Biology: foundation of Genetics (Mendel), discovery of transposons (McClintock).

Studies on the etiology of other plant diseases led to additional surprises as the discovery some 40 years ago of viroids, the first class of independently replicating subviral pathogens, as well as of satellite RNAs.
Although it is difficult to think of a pathogen with a genome smaller than that of viroids (250-400 nt), the possibility exits that new unconventional pathogens may still wait to be discovered (see below).
Therefore, research on etiological agents of plant diseases has been extremely fruitful in the past and this might also be the case in the future.

There is still quite a few number of diseases affecting economically important crops, particularly fruit trees, whose causal agents have not been identified so far.
These diseases are presumed to be incited by virus or viroid-like pathogens, but efforts to characterize them have been unsuccessful.
Only for technical reasons such as the low concentration or instability of the viral particles? For other reasons including a new type of pathogen?

Prions (infectious agents assumed to be composed exclusively by protein) have been discovered in animals and fungi (yeast). Why they could not also exist in plants?

In addition to gain new basic knowledge, research on the etiology of virus and viroid diseases has important practical implications.
Once a pathogen of this kind has been identified and characterized, the way is paved for setting up rapid diagnostic tools for its control (ELISA, dot-blot hybridization, …)

But in this context of practical implications there is still more.
In some cases, the effects of a virus or viroid pathogen are not necessarily “pathogenic”. For example, there are ongoing studies to engineer some citrus viroids as dwarfing agents to reduce the tree size and facilitate crop harvesting.
There is in this session an interesting communication on the use of PLMVd for delaying bloom and minimize the risk from late spring freezes in the southwestern United States.

Together with this communication there are others dealing on the etiology of diseases of apricot and apple, on the interaction of two viruses in a peach disease, and on the variability and molecular typing of three ilarviruses affecting stone fruit trees.