I am not so sure that is rot. There seems to be a very good set of live roots which is not usually present when rot is killing the plant. I think you are seeing the plant shuck the last of last years extra layers, which is kind of normal. As long as you have good new growth like you do and healthy looking roots like these, I would not worry.

I have one of my grocery store varieties that is showing some rot with the tops dying off. It is starting to look like I am going to lose most of that variety, but then it was an experiment in the first place.

But everywhere in my garden I have onion shoots, and all the other garlic varieties are doing very well. Looks like quite a pile of volunteers and one bed of onions that I thought died off last summer and didn't bother to dig, but now they are right there in their rows! Bulbils from Japanese and my feral and the sand leeks (both cloves and bulbils) are coming up real well too. Of course the walking onions are all growing and showing real nice already; scallions very soon I hope...

I suspect that many of the volunteers may be seedlings of garlic chives that self sow all over my back yard. A quick hoe will decide those fates, at least now that it is so early.

Old, I think the grey part in the middle is some kind of rot. It is just that this plant is very large and strong and is still doing ok.

In the same garlic bed, I've seen several plants died and dried up. So I think it was the rot that killed them. But since this was still virgin garlic bed, with bulbils, I do not think this has the dreaded soil disease, but regular rot from the wet soil.

My main garlic bed is on a east facing slope and I hope it will do much better. Wet soil has been my problem. My main garlic bed last year had big rotting problem. And I lost majority of the mature garlic due to spring wet soil.

It also looks to me like it MIGHT just be normal decay of an outer dead layer. I'm not sure why it is dead but any dead plant matter in the ground is going to rot. How does the bulb look if you peel off the dead part?

I had a few plants, with totally wet and rotten stems, just a few of them. But it is still alarming to me. This one probably is probably strong to withstand the wet condition.

Red, it looks to me like what you have is the last sloughing off of the underground portion of the leaves that have already died off on top. The new plant is the vigorous set of new leaves in the middle connected to strong healthy white roots. Together they are the best sign of a strong new plant.