Can't help you with your problem, but you can enjoy your self checking out my face book page. Just dahlia stuff nothing that is not family friendly.

Hi, I am working on a similar problem...is it possible for you to email me the photos you have of the dahlia tuber with the cauliflower like growths. thx vb

Hi there
could be caused by bacteria, and called crown gall. I hope the link works. If your Dahlias have that stuff, it seems necessary to destroy diseased tubers.

Hope that helps, best luck, Lin

Here is a link that might be useful: Crown gall