Are they blooming at all? Or is that what you mean by "shut down the plant" -- in other words, not blooming?

If so, look for holes drilled into the flower buds. the tiny dots are likely frass -- caterpillar poop.

This was identified in another forum and I'll share. I thought it was caterpillar frass, too, at first. Citrus blackfly not a possibility (it's a warm climate pest).

Anyway, we overlooked the biggest clue seen in the first picture. Anybody see it?

The ripe seed pod of the petunia is spilling seeds all over the place!

Petunias should be dead-headed with regularity, or they go to seed. Seed development takes energy from the plant. Petunias also tend to go downhill in the heat of the summer.

Usually, a good haircut and some fertilization will bring some juvenile vigor back to a petunia.

Ok, here's the deal. I took samples to our local Agri office. The Petunia had botrytis blight. It did not have sooty mold. Botrytis blight caused the leaves to drop fast exposing the stems to intense direct sunlight all day.
And those are Petunia seeds. lol. Thanks Everyone for your comments.

"seed pod of the petunia is spilling seeds"
Arrrgh! So much for jumping to conclusions. (sigh)