I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
It depends on what is causing the death. Usually it is dry soil or soggy soil conditions. It could be damage from a weed killer such as one used on a lawn next to the flowers. If you've had lots of rain it may be fungus or bacterial blights. There is no cure for weed killer injury. Petunias are not normally plagued with diseases unless they are rained on or irrigated too much. Thus they generally don't need to be sprayed. Finally if you pull up a dying plant and seed swollen small areas on the roots it could be nematodes there is no cure for them.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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