Those black striped stems sure looks like fungus to me... I read about this some time ago, but can't remember where or what it is called.

I would cut off any growth that has the black stripes, and dispose of it in a dumpster (not compost). I would read the directions of the fungicides, and see if either recommends using it as a drench. Use the brand that had mix instructions for a drench, and follow instructions. I'd repeat this in line with the directions. I would be tempted to apply the fungicide drench to your other healthy plants nearby, to avoid spread.

If new growth emerges with the black stripes, I'd either use another brand/type of fungicide, or dig up the tuber and leave that spot barren for this season. Next year, the problem will probably be gone.

Watch how much water the plant receives. In this case, I'd err on the side of DRY, to keep from spreading the fungus in the soil to your other plants.