What Causes Yellow Leaves on Petunias?

Yellow leaves on petunias are caused by a deficiency in nutrients, fungal diseases, viral diseases and aphids, which are also known as plant lice. If caught early, the damaged parts of the plants can be removed and discarded. If the plant is too diseased or infested to save, the area should be replanted with new plants and fresh soil. Preventative care is the best solution for petunias.