As mentioned by another party, meristem tissue culture is
normally clean, and traditional vegetative propagation from rapidly
growing shoots [tip cuttings] of iinfected plants often is virus-free.
[Obviously, the virus must be symptomatic for any assurance on the
latter.]
The TMV-by-smoke is real enough that no research greenhouse
[except those funded by the Tobacco Council?] will allow smoking, let
alone "butt-infusions" -- if you choose not to believe that, well, most
nineteenth century physicians didn't believe in "germs"!
Finally, one person mentioned human infection by plant virus when
they were at the University of Oregon. Unless this is massive
coincidence, he referred to the "Cordley Hall syndrome" at Oregon State
University [circa 1970]. I believe that only two plant-virus researchers
contracted fatal neural-degenerative diseases at about the same time.
Plant viruses were [and are] suspected, but I know of no
non-circumstantial evidence. Other environmental causes, including a
laboratory ventilation system that sucked the exhaust from one set of
laboratory exhaust hoods into the intakes of another, were also considered.
Still, the traditional "sandpaper test"
[grinding viral material into uninfected plants] was modified so that the
material isn't ground into the researcher's skin. And to add another note
for the paranoid, all of us are likely to be bitten occasionally by
leaf-hoppers, a major vector [after aphids] of plant viruses.
Loren Russell, Corvallis, OR