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.
Petunias should be fertilized every two weeks to avoid nutrient deficiencies. The fertilizer needs to be water soluble with a nitrogen, phosphorous and potassium ratio of 24-8-16. The soil should be lightly watered before fertilization.
Fungal diseases often occur due to humid conditions caused by over-watering. Petunias should be watered to a depth of six inches and only watered again when the top one inch of soil is dry. Plenty of air circulation and well-drained soil are necessities.
Viral disease can only be prevented with disease-resistant soil and the elimination of weeds that harbor pests.
Aphids are treated with insecticidal soap spray, either commercially purchased or made by mixing 2 to 5 tablespoons of dish soap with a gallon of water. This spray should be applied on both sides of the leaves when conditions are cool every four to seven days until the aphids are gone.