[snow white]

One of my Purple Majestys looked to be suffering from what I thought was blackleg,    so I pulled the plant out.  Underneath was a beautiful crop of pots.  All fully grown.  The halum stalk was perfect, in looked like the halum was doing a normal die back. It is a main crop so how can this be?

[simonwatson]

If only one of the crop was doing this, it seems unlikely that it's really natural die-back. After all, if you've treated them all the same, why aren't they all behaving at least similarly. You might have just caught the disease early due to your vigilance.