The top plant is most likely toast, but hopefully the tubers will put up new growth. Luckily, there's still time for the plant to recover and put out blooms this year-- don't give up on them yet! Good call on flushing with water, as nitrogen is water soluble, and you might have saved the rooted tuber under the soil.

I did something similar with a lower dose of nitrogen... Here's my post that has Picts of my disaster: Stem Damage at Soil Level