The petunia is a herbaceous perennial and is often found in many yards and gardens. These flowers come in a variety of colors, such as pink, purple and white. However, the beauty of the petunia is often marred by the presence of black mold on its surface. Sooty mold is common in petunias and often causes the flower to become unattractive.
Black sooty mold is a condition that occurs on a variety of landscape plants, including petunias. This condition is easily recognized by the growth of black, crusty fungus on the leaves, stems and flower heads. Sooty mold is considered a cosmetic condition and does not usually harm petunias. However, petunias with very heavy coverings of sooty mold fungi may suffer from reduced vigor and poor health.
Petunias are susceptible to infestations of sucking insects that can cause sooty mold to develop. Aphids are common petunia pests. These tiny insects congregate on the undersides of plant leaves and are typically red, orange, yellow, pink or green. Aphids feed on the leaves of petunias by using their sharp mouthparts to remove plant sap. Consuming large amounts of plant sap causes aphids to exude a substance known as honeydew, which causes sooty mold fungi to adhere to the surface of the host plant. The presence of sooty mold is a sign of heavy aphid infestation.
Controlling insect infestations is key to reducing black sooty mold on petunias. Aphids may be removed from petunias by spraying them with water twice each week. The forceful stream of water knocks aphids from the plant, reducing their numbers. Remove small numbers of aphids from petunias by dabbing them with an alcohol-soaked cotton ball or cotton swab.
Heavy aphid infestations may require insecticides available at your local garden supply center. Horticultural oils are often used to eliminate aphids on landscape plants. These products work by coating the aphid with oil, causing it to suffocate and die. Horticultural oils must come into contact with the insect to kill it, so be sure to apply these products thoroughly. Concentrate on the undersides of plant leaves, where aphids gather to feed. Insecticidal soaps are another product that is often used to eliminate and control aphids. Some plants do not tolerate insecticidal soaps, so be sure to test these products on a small area of the plant before applying it to the entire plant. If your petunias show no ill effects after seven full days, it is safe to use the product on the entire plant.