Drought-tolerant oleander (Nerium oleander) produces stunning blooms and can grow up to 8 feet tall. This broad leaf evergreen grows in U.S. Department of Agriculture plant hardiness zones 8 through 10. Although it has no serious pest or disease problems, oleander can develop a black sooty fungus on its leaves, known as sooty mold.
1. Sooty Mold
For sooty mold to grow, the oleander must be infested with sap-sucking insects -- such as aphids or scale insects -- that produce the sticky substance known as honeydew. Sooty mold will grow on the honeydew, leading to a black mold forming on the tender portions of the plant. Sooty mold is rarely a serious disease, but it will negatively affect the appearance of the oleander. In extreme circumstances, the sooty mold can cover entire leaf surfaces, blocking out sunlight and interfering with the plant's ability to photosynthesize.
2. Controlling Honeydew-Producing Pests
Before you can take care of the sooty mold, you must first control the sap-sucking pests secreting the honeydew. Neem oil is a safe and effective pesticide used to control honeydew-producing pests, such as aphids, leafhoppers, mealybugs and whiteflies. It won’t harm beneficial insects and has little to no threat to people and pets. Spray ready-to-use neem oil directly on the leaves of the oleander to the point of runoff. The honeydew-secreting pests must come in direct contact with the neem oil for it to be effective. Although neem oil is a natural product, you should still wear long sleeves, long pants, protective eyewear and keep children and pets out of the area when you spray.
3. Remove Sooty Mold
Once the pests are gone, you can wash the sooty mold off the oleander with a garden hose. For smaller areas, wipe the mold off the leaves with a soapy washcloth. Be careful when washing or wiping the sooty mold off the leaves not to accidentally damage the foliage. This damages the plant and the sap from the oleander is toxic. Alternatively, allow the sooty mold to wear off the plant itself. The elements -- such as rain, wind and sun -- will help get rid of the sooty mold without your help. It could take several weeks for the mold to go away on its own.
Keeping the oleander healthy and vigorous with proper cultural care will help prevent future problems, such as pest infestations. For example, regularly remove weeds growing near the plant. These unwanted plants make the oleander compete for water and nutrients and provide a hiding place for honeydew-producing pests such as aphids and whiteflies. Before using insecticides to control the honeydew-producing pests, read and follow the directions and warnings found on its label. This will help prevent damage to the oleander while increasing the insecticides’ effectiveness. All parts of oleander are extremely toxic and can kill a child or pet if eaten.