I have read it is spread by mites, so if you don't have mites... Of course there are other sucking insects too, like aphids. Don't know if they are carriers however. Be sure to also sterilize pruning sheers between plants.
I got one batch of cuttings of Mary Lane seedless that are so infected the leaves look as if they have been hit with round-up residue - very small and very gnarled. They are offensive looking. But I'm going to try hanging in there with them - if they have the strength to survive. Many other new cuttings with leaves show evidence of virus too, but to a much lesser degree. Mostly mottling and a bit of distortion here and there, but they at least look like leaves.
How sad that a disease such as this is so extensive and untreatable. :(