Uh oh Cici. So sorry. I lost one of my dahlias too. Luckily the tuber had one more healthy plant growing from it. Mine was not due to fertilizer but too much rain. It flowered and then the plant started rotting :( I lost a columbine though because of my tlc :)

Awww, CiCi. I am so sorry. I lost one last year due to improper staking. It was heartbreaking.

Thank you for the pictures, helps me confirm some naughtiness I have done with some of my other plants.

The images I posted shows the results of tossing about a tablespoon
under the foliage half-haphazardly, often settling too close to the
stalk. I had lost quite a few 3" pot roots and destroyed about a dozen plants (out of 500) through that piece of stupidity, while others stopped growing for several weeks while recovering from the overload. After carefully sprinkling a tablespoon of nitrogen under the
first 100 plants, I had got sloppy with my aim for the 400 remaining
plants. I will be sure to not get as close to the plants this year,
guaranteed!

I was talking to a master grower last meeting about my 'faux pas' with the nitrogen application, and he said to gently rake it into the mulch/soil about 12-18 inches away from the plant roots, at the 'drip line' of the foliage. According to him, raking in the nitrogen (urea or other type) will force it into the soil instead of evaporating into the air, and the root system will take in what it needs. The unneeded nitrogen will be leached out of the soil within a few weeks.

It's easy to overdo a good thing.