Disease

• One of numerous diseases could be behind your dying petunias. Fungus infections such as Botrytis cinerea can cover the plant with a silvery coating and eventually kill it. Insects such as aphids also can spread disease, such as mosaic viruses, from one plant to another.
  
  

Care

• Death in petunias can be caused by inadequate sunlight and water. Petunias require about six hours of bright sunlight per day. Water them weekly if there is no rain. Too much water can make petunias more conducive to fungal infections.
  
  

Considerations

• Cold weather, especially if it is cold enough to frost, will kill petunias. Soil conditions may be not be appropriate, either. Using a balanced fertilizer on petunias may help keep them alive.