Are you watering them?
Linda C

Are they getting too much water?

Are you making sure you are deadheading them, pinching them back as necessary? The reason I ask is, I recently had to purchase some Merlin petunias from Wal Mart, as all my seed grown ones were devoured by rabbits. I noticed that all plants had never been pinched, they were these long, leggy plants with one flower atop a 10 inch stem. There were many plants in the 6 packs that were dieing out, and there were dried up flowers all along the stems. Many plants were dead. I am sure there are 100 other things that could lead to the death (no doubt improper watering), but I am curious if a petunia that has never been deadheaded will eventually die out. Anyone know??

A petunia will continue to grow until frost......even if never deadheaded...It will even produce some side branches, but not as many as if deadheaded regularly and pinched back when a stem gets long...
Linda C

I have had this same problem, now for the third year, after having beautiful ones before. The problem seems to be soil-borne and from reading other sites, soil fungicides will do little good. The best suggestion is to plant something else for a few years -crop rotation! Apparently Wave petunias are especially susceptible to this problem.

Sounds like a possible Ralstonia solanacearum AKS Southern Bacterial Wilt. Host plants include those in the Solanaceae family which includes Petunia, Tomato, Pepper, Eggplant, Potato...weeds like Ground Cherry, Datura,and the Nightshades.
Spreads via soil water from plant root to plant root...does not show much in the way if any of water-soaked leaf-spots as do other Bacterial Leaf Spot pathogens like Xanthomonas spp. Plant can be infected in cool weather but not show any symptoms until the weather is warmer...yellowing of lower leaves, sudden wilting even with good soil moisture.,etc. The bacteria enters through the roots and plugs the vascular system and therefore inhibits the uptake/conduction of water/nutrients.

I would talk to your local extention office agent and see about a positive ID

Vera