as i understand it ... you bought these pre-planted ... which to me means OVER planted ... for fast sale and immediate gratification ... if they are flopping every night after being in the blistering sun during the high heat of the day ... that means the roots are too jammed in what little potting media there is ... there isnt enough media.. to hold the water they need the ONLY solution.. FOR NEXT YEAR ... is bigger permanent pots ... with enough media ... to sustain the plants for the season ... and just enough plants to grow in such .... but you will have to wait a month or two.. for them to fill up the pot ... i would bet my shiny nickle... if you took the pot apart.. and tipped out the root mass.. you would be hard pressed to find any media.. to hold water.. for the whole day ... these pots need to go in shade.. during the heat of the day.. and you will probably find.. that they can actually making it thru the day ... without looking like a bunch of wilted cut flowers ... yes.. i know.. that defeats the purpose you bought them for... but them dying does also ... put them in the patio in the yard in the shade.. and you can enjoy them for the rest of summer ... ken

It's impossible to know for sure with the given information, but I would strongly suspect a soil moisture problem. Watering a potted shrub "every week" is not gonna work! If you watch what nurseries do, they are watering their plants EVERY day in hot weather like we are having. You might get by with watering every other day, but not once a week! If you are using water retaining gel, or something like that to extend the time between watering, then you almost certainly have a serious drainage issue, even if the top of your soil appears to dry out quickly.

Hi, Alisande! Unfortunately, some herbaceous plants like petunias can be affected by an assortment of fungal diseases, the spores of which are ubiquitous. Why some become infected while others don't......who knows? (Look up white and gray molds, powder mildew.) I might be able to tell which of several fungi (or other issues) might be the problem if I could inspect the plants in person, but frankly, my own personal solution would be to remove the offending plants entirely and plants something entirely different. I'm not one to reach for a broad spectrum fungicide for an issue like this, where the problem seems isolated. After the end of the season, be sure to dump all of the potting medium, wash the pots, and maybe give them a diluted bleach/water rinse before storing.

The little brown specks are seeds. When you don't deadhead the flowers the spent blooms will form seed pods, which will pop when they are ripe hence little tiny seeds go everywhere. In order to get more flowers instead of seed pods you need to deadhead the spent flowers. If I were you I would cut all the stems back to about 4 inches long and fertilize the plant with a liquid fertilizer where the middle number is the highest number. The middle number is phosphorus which affects blooming. When you use a fertilizer where the first number is the highest number that causes a lot of foliage growth instead of flowers. Make sure the plant isn't dry when you fertilize it. The plant will look dismal after you cut it back but never fear it will grow back out especially with the addition of the fertilizer. You should fertilize petunias with a liquid fertilizer once a week, but never fertilize them when the soil is dry. Deanna