Petunias can get yellow leaves from lack of water, too much water or lack of fertilizer. If they are wet and you have them in pots or not planted yet, then put them in a protected area and let them dry out a bit and then give them some half strength fertilizer--miracle grow. While you are waiting for them to dry cut back the tendrils by half and pick off any seed pods. This will cut down on the stress of the plants and make them bush out.

The white looking stuff may be fertilizer residue, but keep an eye on them, if it starts to spread treat them with a fungicide for powdery mildew.

Your soil may have a time release fertilizer. It should say on the bag the fertilizer numbers, something like 7-10-12 or 12-12-12. If it says feeds for 3 or 4 months then that's probably what is in it and no other fertilizer is needed. I would still give your plants a shot of fertilizer to start them off though. It takes a little while for the time release to start working.

Lastly --yes you can post pictures--if you don't know how--ask and I'll tell you how

Oh thank you so much for your reply.... I think I may be in trouble because they were moist and I repotted them into my baskets and watered them (I added potting soil) with miracle gro .... oy
I took pictures before so if you could tell me how to post i would love it. I'm worried about this "powdery mildew" b/c I bet that's what it is.... This guy told me "nope it's just what happens to the flowers when they die" but there's definitely a "puff" of something when I pull them off....
I'm so mad! (and so stupid...!)
Thanks again....

To post a photo--
Get a photobucket account--it's free. Upload your photo. Under the picture you'll see a bunch of URL formats for the picture. Click on the one that says HTML. It should automatically copy. Go to the thread here and right click and choose paste. It will show up as writing but when you submit it will be a picture.

That's ok-- they may just have needed fertilizer to take care of the yellow leaves. Let them get fairly dry before you water again. Set the pot in the sun to help dry it--somewhere where it will be shaded in the hottest part of the day.

Give them a hair cut too. Trim back the trailers by half. Don't worry they will make more trailers but it will bush it out too. Now when you say pull them off do you mean just the dead blossom? Yes that puff could be from the dry, dead blossom. You dead head by nipping off the little ball behind the blossom--that's the seed pod.

Powdery mildew will kill the leaves and it will spread. If the leaves that had the white are not getting any whiter or brown on the edges it may be nothing. If you want to be on the safe side spray the leaves with a fungicide. Do it with the plant in shade or in early morning or early evening. You want the plant dry before nightfall.

A picture is worth a thousand words. Yes, you can post on here and the easiest way is to go to a site like photobucket and follow their directions. If you can't figure it out then, let us know.