I bought four of these from a reputable nursery and planted in front
porch planter box last year. They did very well for months, until
(about 2 months ago) I noticed the far-right one was turning black (no
political implication intended g This spread from R to L until all
four plants were completely black or getting that way.

I took them back to the nursery today. They were very nice, examined
the plants, looked up info, but came up with no clue as to what was
happening. AFAIK, I gave them proper water and they got sun 3/4 day
in summer and 1/2 day in "winter".

I was concerned that the soil had been infected, but the other plants
seem fine.

Is this too fragile a plant?...Or...? Never saw anything so strange
as the R to L progression.

Any clues?

TIA

HB