There is a disease that would effect both the azalea, and the rhododendron. It is a viral disease, with a soil born pathogen. It is pretty easy to tell if that was the case. The leaves on both
plants would have rolled up tightly like a cigar and turned brown, then
plants would have died. It should not have harmed your petunias however. It could be could be just a coincidence that the
flowers died, by some other cause. If it was the viral disease that took out your rhododendron and the azalea, don't
plant the same type of
plants in that location again, for at least 5 years. Then only try a cheap one 1st to see if the pathogen is still there.
It is possible that the
plants died from over watering, but I have no way of telling. It has been a very hot dry summer here as well, but also extremely humid, and the soil stayed damp for weeks after watering. Not to many insects will infect azaleas or rhodies ,at least to the point of killing it in one season. So I doubt that is the case. Also while both are very prone to developing leaf spot and anthracnose, those diseases usually will not damage those type of
plants to the point of dying in one season.
One other possibility could be a natural gas leak, but that should have killed everything in the one area, and not left anything alone.
Hope this helps.
Mike
