Doubt very much if you would find a way to prevent the white patches appearing on your red petunias .....they look beautiful to me so I would just enjoy these delightful flowers.

Some flowers do produce variations like this. Nurserymen sometimes take advantage of it and us the plant to propagate plants for a new variety. There isn't anything you can do to change them and its very unlikely that the others will develop the same variation. if you really don't like it you could try removing the affected pants from the container but at this stage of the summer it would be a bit dangerous if you wanted to replant them elsewhere.

Thanks to you both. I just thought it was strange that the white patches only developed on some of the flowers on the same plant. I thought perhaps it was a stage the flower went through before dying or something like that.