I don't think you did anything wrong. Looks like a disease of some sort to me.
I would just try again in another pot with different soil and make sure that there are drainage holes in the bottom. I don't know about Canada but where I live in California in zone 9b they will grow and reseed themselves in the sidewalk cracks. Lol. I must say that I never have grown them in pots. Just in the dirt outside. Can you try them outside in the garden?
I wish you luck. You'll do it.