I always have the same problems with petunias... They grow very nicely at first, get big and pretty, and then start to turn yellow like they're chlorotic, missing minor elements, or have been overwatered. Once that happens, a fungus seems to get them because the leaf edges turn brown. Before long, it's a sad, weedy thing that lives seemingly forever as the majority of it dies off, with one sad shoot surviving. Then that shoot dies just after another pops up. Eventually I give up and rip the eyesore out.
I do recognize that they are annuals. But out front I have a very nice bed of petunias, about 8 plants worth. They receive 4-6 hours of morning sun and then go into shade. 7 are big and beautiful with perfect green foliage, but in the last few weeks one of them is starting to turn yellow and is getting brown on the leaf edges. If it were just them dying off in their annual cycle, I would expect it to be happening to all of them since they all came out of the same tray and were the exact same size when planted.
Any ideas on what exactly is going on?