It may be too wet.
One suggestion would be to dig it up and take a look at the tuber. If it is rotting, sometimes you can save it by trimming off the nasty part. That is presuming that it's not the eye part that is bad.
Suggestions from anyone else??

I'm with Granny on this one having suffered the same fate about a hundred times over. If it's wilted and recently planted, I'd suspect too much water and the tuber is rotting. If it wilts in midday heat and bounces back in a cool evening, then you're okay.
I'd not water any more for a while, and if the dying/wilting continues, gently dig up the tuber and do what Granny said to try to salvage it. It can be done with even an inch long stub of clean tuber, just takes longer to establish itself.
Good luck, let us know how it turns out.

The tuber put up another sprout but it quickly died. I dug up the tuber and it was rotting, so I cleaned it and put it into a pot.
New question, other sprouts look 'wilty' and I'm going to assume they may also be rotting, should I dig them up and put them into pots? We had over 10 inches of rain in the last 2 weeks, the ground is saturated.
At what point do you not have to worry about tuber rot?
Thanks,
Chigger

Chiggerbait, love your name by the way...

Your weather sounds like ours in about January- not ideal for dahlia tubers newly planted in the ground. If they continue to look wilty, I would indeed dig them up gently and see what's going on. My past experience with 'wilty' leaves has almost always been rotting tubers. Worse still: leaves that never appear at all. So sorry. BUT all is not lost yet; you can trim them and try to save them as above. Make sure you cut away ALL rot, let the cuts heal over and it wouldn't hurt to treat with a fungicide either. I've had dahlias go on to grow long after I gave up on them as totally rotten. When the tubers were dug up that Fall, the original tuber was nothing but a stub- half an inch of neck. Yet they went on to produce a full grown productive plant/flowers. Who can't like that?

Putting them in pots to let them regroup until your soil dries out a tad might be worthwhile too. Or leave them in pots- then you can protect them from hideous rainfall but will have to water more often in dry spells.

When to quit worrying about tuber rot? Right around October. There is always something wanting to foil your best dahlia plans.

I can't use sulphur or fungicide - bad allergic reaction one year - so we spray the cut ends with Lysol - it works wonders. You can also use it during the winter if you spot fungus growing on any tubers.