There isn't an insect or disease that specifically targets and kills petunias the way you're describing. The pubescence (fine hairs) on petunia foliage offers some protection from insects and disease, as well. I'm suspicious of your soil. Could it have some kind of toxin in it--perhaps runoff from the street or driveway or excess fertilizer? If you're growing in a container, throw the soil out and buy new. If you're growing in the ground, take a sample in to your local university extension office to have it tested. Also, make sure the spot drains well and the soil doesn't stay soggy wet after rain. Another way to test the site is to try something besides petunias to see if it dies, too.