If it was later in the season, and took longer then two days to turn yellow, I'd say water in some Epson Salts (for magnesium) and lawn fertilizer (for slow release nitrogen) to green it back up. Was there a lot of rain recently? Both Magn and N are highly water-soluble, and could have been flushed from the soil. Did you plant it right before the new yellow growth came out, and this new growth is responding to a deficiency in the soil?

i was hesitant to reply to this post, as I have never seen yellow happen so quickly, and to such an extreme, in two days. It sounds like your other dahlias are just fine. If the tuber was rotting, the top would slightly wilt before plant dies, as rot moves up the stem. Virus would have a more streaky yellow throughout the plant. I feel I'm missing something important to this puzzle.