Several diseases and pests plague petunias, usually if the petunias become malnourished because of the soil they are planted in or watering issues. If plants that attract certain pests, especially aphids, are near the petunias, and those plants are infected, the pests may move over to the healthy petunias. Pest and disease control throughout the garden is key in growing problem-free petunias.
Phytophthora causes crown and root rot diseases in petunias and other bedding plants, particularly if the soil is too moist for long periods of time, or if the petunias are planted too deeply, according Utah State University Extension. Control the pathogen by making sure the soil is well drained. Phytophthora starts in the roots, travelling up into the root crown, then into the stem. It affects one part of the plant at a time, and progressively attacks the whole plant if it is not controlled. Leaves on the affected parts of the plant turn light green or yellow. Use fungicides on the petunias, in conjunction with the cultural controls.
Aphids are small bugs--ranging in size from 1/16 to 1/8 of an inch long. They may be black, brown, green, pink or red. The aphid's body is pear-shaped, and it has slender antennae. Aphids are usually found on the undersides of leaves, but you may see some flying around, or sitting on the top of the leaves. Aphids prefer new growth. They suck the sap from the underside of the leaves, which causes the leaves and stems of the petunia to become distorted. Sooty mould grows in the honeydew left by the secretions from the aphids. Aphids can be controlled with insecticide, or you could pick them off by hand and wash them off with a steady stream of water.
Rhizoctonia solani is a fungus that shows up as a thread-like growth on the petunias. The plant shows Rhizoctonia solani with the appearance of reddish spots that expand, and eventually kill the plants, according to Oregon State University Extension. The fungus grows fast--up to 3 1/2 inches in three days. The pathogen lives in the soil, and can persist for years, attacking and killing young plants. Any seedlings that manage to survive have root rot. Control Rhizoctonia solani with fungicides.