Julie,

It’s not real easy to see but this damage on your petunias appears to be castings from leaf hoppers. Leaf hoppers can damage plants by biting and sucking out juices. Your petunias may not recover fully given that they are annuals so if you over look this and just enjoy the blooms do so.  I’d remove these plants in their entirety and throw them away. Do not compost.  You can try to rinse off these leaves but any holes or dark spots will remain.  
A product you can consider is diatomaceous earth. It is a dusting powder can can help but it may too late.

 Clear out the area and mulch it well if you do decide to pull the plants now.  Let the ground rest until next spring. 

This link will give you more information.  
https://www.canr.msu.edu/ipm/diseases/grape_leafhopper

Thank you 

Barb