Answer: It sounds to me like there may be some sort of disease at work on your petunias. A lot of petunias are disease resistant now, but not all are, and to say a plant is disease resistant isn't saying it's disease proof. I suspect a disease because the petunias are succumbing exactly the same way at the same point in their growth. I am further convinced of a disease because you tell me that they are receiving adequate lighting and water. My suggestion is this: Rather than try to save the petunias you have, I would remove the petunias and the soil from the pot and throw them away. Sterilize the pot with a solution of 1 part bleach to 10 parts water and/or run through the hottest cycle of your dishwasher on the heated drying option. Start from scratch with new, sterile potting soil (throw away any that is left in the old bag) and new petunias.
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