Answer: Sounds suspiciously like Verticilium wilt, which normally attacks tomatoes, but because petunias are in the same family as tomatoes, they are also susceptible to Verticillium wilt. If the cause is indeed verticillium, you should change the soil and clean the planters thoroughly with a 10% bleach solution before replanting petunias. For now, I suggest replacing them with a different species of ornamental, and starting with new, clean planters and soil. Use bagged potting soil that is meant for planters; do not use soil from your garden, since that's the likely reservoir of fungal spores. For replacements, avoid these succeptible species: snapdragons, cosmos, china aster, bachelor's button, dianthus, and stock. Good luck!
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