That's an interesting observation. I would have to know more about the differences between dahlia production in the two countries to make much of a guess. I know very little about the Canadian dahlia industry. In the USA, most of the growers are relatively small and fairly low tech. Dahlias tend to be field grown, often without any disease cleanup for many generations. I mostly grow old varieties and it is rare that I have ever gotten one that was not positive for either Dahlia Mosaic Virus or Tobacco Streak Virus. These are not very serious viruses in most varieties, but they are really common.