I tried searching for that and nothing showed up. I’ve been getting rid of the plants that look infected. I might have to uproot all of them. But I’d prefer not to.

I tried searching and didn't find anything either. Hopefully cutting it back rather than uprooting will be good enough

It looks fungal to me too, but the closest damage match I could turn up for Asclepias spp was fusarium, which would collapse the interior structures of the stem, rather than looking like huitlacoche like this