James:

Never, ever grow the same plant in the same spot year after year. If you get a disease one year you'll get it the next year.

We always rotate crops/plants which reduces chances of diseases and/or pests.

I would plant marigolds in another spot. We grow Crackerjack and it always produces tall plants with large gorgeous flowers.

Never ever is a little extreme I believe. I believe this blight would have struck where ever they were planted but of course I could be wrong. Though I do try to learn from my mistakes.

I plan to plant black-eyed susans -Rudbeckia fulgida- the Goldstrum strain in this place. I already have 400 potted and I'll need more room.