About 30 years ago when I grew bearded iris, growers in the Eastern part of North America would use milk to treat a soft rot of the rhizomes. As far as I know, this is not a problem in Western North America, so I have not had any personal experience with it. That is the only use of milk against plant disease that I have heard of. I did not get in on the very beginning of this discussion as I deleted what I thought were messages about computer viruses, so I hope I don't repeat what has been said. I think if a plant is very vigorous, it might not show symptoms, but if it becomes stressed, the virus then is revealed. In lilies, some very robust ones carry virus without showing it, and insect vectors can spread the virus from them to susceptible ones that readily show symptoms. I haven't heard of virus being spread through soil. Diane Whitehead, Victoria, British Columbia, Canada maritime zone 8 cool mediterranean climate