Why are my petunias dying?

Question

Plants are planted in a pot with other plants that require watering twice daily. Daytime temperatures reach 100 degrees

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Answer 1

Petunias like it on the dry side. Yours may be suffering from root rot due to too much moisture. I'm convinced that more plants die of too much water than too little but it is hard to judge watering for containers in very hot weather. Here is an article that discusses diseases that affect petunias.
https://www.gardeningknowhow.com/ornamental/flowers/petunia/yellow-leaves-on-petunia.htm