That is an awkwardly placed rot.

you would need to cut off all diseased material, the remaining rest should be white looking, no traces of brown rot.

let dry for 2-3 days, replant in fresh, soilless potting mix,

go easy on the watering.

if there's an eye you can grow a nice plant from a small bit,

but in this case chances are slim, I guess

was it that way when you got it or rather later? Do you water properly that is, sparingly?

Good luck, bye, Lin

I might have overdone it one watering cycle. It's the only one of my tubers that has rotted though. Maybe I'll try cutting most of it away.... there are parts that still feel good but it's such a concave hole.

I would recommend trimming the bad sections off FLAT, instead of scooping out the hole. Keep the cuts clean (wipe blade after every cut), and spritz the final wound with Lysol or other quick-drying antibacterial liquid. Sprinkle cinnamon or elemental sulfur on the cut section, and check for any 'squishiness' after a day or two. Plant when there is no moisture from the cut area.

Should be fine, as long as the neck has even a small white part still connecting the head with eyes to the tuber.

:( I don't think the neck has any white part attached left. I didn't realize how QUICKLY they can rot! I am feeling discouraged about my dahlias... they are such a challenge!

Send that image to Swan Island, letting them know that the neck was all rotted, too. They will treat you right.

feeling discouraged:
Chin up! Dahlias can be approached at any point on the below scale:
Throw in the ground<<<<<<part-time hobby, learning>>>>>obsession, vast collection,
each year like an annual general gardening skills social life focused around it

Most folks on this forum are somewhere in the middle of that scale, which is really cool... anything you learn about dahlias can (for the most part) be transferred to most other plant species. Just like if you REALLY understand tomato-growing, you will be much farther ahead growing dahlias (tubers not-withstanding).

Ok! Trying to stay positive. Trying to grow these for my early August wedding though!

I come from growing other kinds of bulbs and iris rhizomes. Iris are a medium challenge, but not really that bad. They can take a long time to establish. Other kinds of bulbs and corms seem to grow roots so quickly once moistened. Dahlias though seem like they take a very long time to develop roots and are so susceptible to rotting, and take so long to sprout! I am feeling like a fish out of water.

It may be zinnias and sunflowers at my wedding, haha.

Ah, yes. No pressure... ;-) Not the optimum conditions for enjoying the process of learning a skill.