Apparently lots of dahlias are virused, but don't show signs unless stressed. And some are simply more susceptible than others Yellow spots on the leaves, yellow veining in the leaves, leaves that are twisty instead of flat, and stunted compact growth are all signs of virus. One problem is that some of those are also simply signs of nutritional needs, or the wrong ph in the soil.
I tend to only toss mine if the have multiple signs. My Valley Porcupine was stunted, with yellowy twisty leaves. I think I pulled one other plant this year, there are others that are suspicious, but have the right kind of growth pattern and are blooming well, so I will keep them and see if they show the same signs next year.
What I know about it all is really minimal, I am just spouting off what I have read from Ted of Hollyhill Dahlias, who is a regular on the Dahlia Cubit.
