The only way to be reasonably sure of a virus-free collection is to test
each and every acquisition, using ELISA techniques for example, for viruses
known to infect that particular type of plant. This done after the entire
collection has been tested and any infected plants disposed of preferably
by incineration.
A second best, and perhaps more practical, approach is to segregate plants
based on their source. If one shows any signs of virus, either in
unexplained deformities, declining vigor, color breaks, etc., the entire
group should be considered suspect, and possibly destroyed.
I know that in some orchid collections (my own particular speciality), more
than half the specimens have been found infected, although not showing any
obvious symptoms.
I want to hit on this, because it is so easy to overlook. Aphid
infestations are pretty obvious, and bacterial fungal infections can also
be relatively easy to detect. Virus sometimes is not, and can be easily
spread through exchange of plant divisions/propagations.
Carl