| If the black substance is on the surface of the leaves and can be scraped off, it's sooty mold which is the result of insect feeding. Aphids and other sucking insects excrete a sticky substance when they feed. This substance is called honeydew and when it ages it turns black. To keep the sooty mold from developing, you'll need to control the insect pests. Check your tree periodically for aphids or other insects. If you find any, you can control them with sprays of insecticidal soap or you can use a systemic insecticide such as Merit. Be sure to apply according to label directions. |
