Answer: Based on your description this sounds like it might be Phytophthora, a soil borne disease which can cause sudden wilting and collapse. If you are growing these in containers, be sure to start with clean tools and clean containers (scrub clean then dip in a solution of one part bleach, nine parts water)and be sure the drainage holes in the containers are working correctly. Every year, use fresh clean soil mix -- use a soilless mix formulated for containers. Soilless mixes contain peat moss or bark with perlite and/or vermiculite for example rather than topsoil or soil. In the garden, grow the petunias in a different spot every year. Also be sure the soil is well supplied with organic matter and also well drained, as overwatering or waterlogged soil will encourage the Phytophthora. Your local county extension may also have suggestions. If it happens again, I would suggest you work with them for a more specific diagnosis based on seeing the affected plants. In the meantime, I hope this helps.
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