After much poking around & pulling up onions & garlic, I think I just have regular soil fungus & some overwatering and not any specific pest. Eesh, I need much more practice with this growing onions thing.

Is there mulch in the bed? If it is all in the soil it is probably just decay mold from overwatering. I do not think white rot is common.

There are so many kinds of rots. It is hard to accurately ID them.

I believe White Rot is not uncommon in some areas of California. I seem to remember reading that many garlic fields in the Gilroy area were rendered not usable for garlic due to white rot in the soil.

Here is a link to an article that mentions white rot in some Gilroy fields.

White rot

And another article on a possible solution for white rot.

White rot remedy

Can White Rot happen by itself? Or through seed stock?

There's onions & garlic in that bed, the birds took everything else. White rot exists in nearby farm fields and I get their dust when the wind is right. I think it was just from watering absentmindedly, everything looks better now that I knocked it off and the soil below the surface is no longer a giant patch of white fuzz. Someday I'll get better at this.

@Redsun. White rot can be transferred in seed stock. According to the second article I linked the fungus appears to require a allium host for it to come out of dormancy and then it multiplies rapidly in the soil. Once the fungus is in the soil it can lay dormant for years until alliums are again planted. Bad stuff!!

So then the soil would only be unusable to alliums?

Yes just alliums.

What if someone grew, say tomatoes, in an area that they didn't know had White Rot, then composted the plant(cold compost pile)...that would probably transfer the disease into the compost pile, right?

Yea I guess there would be some soil attached to the tomato roots that would harbor the dormant fungus.