Originally Posted by Thunder Chicken
I don't know much about treatments, but things you can do are limit your watering, and only water early in the day (gives the leaves a chance to dry before nightfall). Try to keep the leaves dry, period.
Rain and humidity won't obey the above rules. If the plants are in a shady spot with poor air circulation, that certainly won't help matters.
Someone here might be able to describe some treatments, but the best solution is to look hard at your plantings and see if the location is appropriate for them. Petunias really like lots of sun and so if they are getting mostly shade they won't thrive and the certainly won't shake off fungal infections.
The good news is that the end of the growing season is close at hand, and early fall is a good time to move petunias. Look around for a sunny spot in your yard that might be a better place for them and transplant them soon.
Gardening is just like real estate - location, location, location.
Remember that many mold spores that take on leaves hide in the soil. If you are watering with hose at typical household pressure and flow rate, you may be splasing the mold or fungus spores as you go.
Think about installing drip irrigation next year. You will love it and your plants even more and it works in gallons per hour. You garden hose and sprinklers you attach to it work in gallons per minute.
Given you notice a mold or fungus problem, clean your garden as best you can this Fall and bag all leaf and lawn waste.
Here in the city, by the way, mold counts prompted an air quality alert a few days ago. You are not alone if that is at all comforting.