Fick recommends giving your petunias an early summer and mid-season dose of fertilizer for blooming plants, higher in phosphorus than nitrogen. Petunias are susceptible to a few virus diseases that can't be cured. Aphids carry cucumber mosaic virus from infected cucumber plants to petunias. Tobacco mosaic virus can be spread to petunias then tobacco users handle the plants. Both these diseases cause petunia leaves to turn a mottled yellow-green and become deformed.
Blossoms are also deformed.
To help protect your petunias, Fick recommends planting disease-resistant cucumbers in the vegetable garden. If you smoke or chew tobacco, wash your hands before working in the petunias.