Thank you for your question on leafing out of your dahlia plants. I am engaging some experts in our group and will get back to you as soon as they reply with thought/suggestions.

Thanks for your patience.

Ruth, Weld County CMG

Thank you for your patience.

My expert colleague thinks this is a bacterial issue with your dahlia tubers. She is not completely sure that it is leafy gall that is caused by the bacteria Rhodococcus fascians or if there is another bacteria at work. These types of bacteria can hang around on the leaves and other parts of the plant and, when the timing is right, will cause issues at the crown of the plant. There is also a remote possibility that this is crown gall but leafy gall is much more common.

Overall, if it is leafy gall, it’s a good idea to throw the symptomatic tubers out as the bacteria could spread to other nearby susceptible plants and other dahlia tubers during storage. It also apparently spreads easily in water so more reason to keep these tubers away from other plants and to keep plant surfaces from being wet for prolonged periods of time.

You could check with the Jefferson County Plant Diagnostic Clinic if they can test to identify exactly which bacteria this might be. As above, isolate the tubers with symptoms away from the rest of your tubers and other plants while you wait for diagnostic information. Their contact information is: https://jeffco.extension.colostate.edu/horticulture/clinic/

Here are the references my expert colleague used. Hope they will be useful to you when raising more dahlias: 

This article from Greenhouse Growers talks about how bacteria creates symptoms of hormone imbalance, leading to tufts of prolific growth. The picture you provided does seem to look like this from my colleague’s experience with growth regulators.

https://www.greenhousegrower.com/production/crop-inputs/how-to-prevent-leafy-gall-before-you-lose-plants/

Here’s another reference from Oregon State that discusses leafy gall more generally. Pretty informative.

https://bpp.oregonstate.edu/sites/agscid7/files/bpp/attachments/is_it_crown_gall_or_leafy_gall_for_web.pdf

Finally, here is a resource from the National Dahlia Collection that shows a picture of leafy gall on a dahlia tuber for comparison: https://stunningdahlias.wordpress.com/tag/national-dahlia-collection/

Hope this helps. Please let us know if you have any other questions.

Ruth, Weld County CMG