If you are lucky there will be a few final Dahlia blooms in your garden this season but it is time to plan for digging and storing your tubers for winter.
Check out SEMDS YouTube video on DIGGING, DIVIDING , and STORING Dahlia Tubers https://www.youtube.com/watch?v=0icqxGVnOlI
Preparing/Storing Tubers over Winter Tip Sheet http://www.semds.org/wp-content/uploads/2017/05/Preparing-and-Storing-Dahlia-Tubers.pdf
If in doubt, throw it out! Learn more about Dahlia viruses from ADS https://dahlia.org/wp-content/uploads/2018/01/VirusBrochureJune2015.pdf