I had a Superbum do the exact same thing to me when I lived in zone 9 - Sacramento, CA. See the picture below of how well it grew from seed in less than 9 months.....

And this is what it did the next year after wintering in the ground.....

It eventually choked while exhibiting the same "black death" on the leaf margins and dug it up to investigate and this is what I saw......

I hope this helps somehow...Jason