Have you been feeding them, Ronf?

You appear to have added an answer to Tugbrethil's question beneath your original question. If you've not been feeding them anything, that will be the cause of the problem, or at least contribute hugely. Summer bedding like this usually gets fed at least once a week right through from planting till the first frosts, when they get binned anyway. I feed my surfinia petunias twice a week with Miracle Gro general purpose mixed as a liquid feed, and have been doing that since end of May - they are planted in containers.

Yep, petunias can survive without feeding, but survival is about all that you could call it. Just for future reference, Ronf, if you put further info in as an answer to your own question, everyone who has commented will automatically be notified. Not so (I believe) if you simply edit the question.