What is sooty mold?
Sooty mold is a fungi that breaks down honeydew or naturally exuded plant materials as their source of nutrition. Sooty mold covers plant leaves with what looks like a dirty layer of black paint, blocking light and limiting photosynthesis. Honeydew excreted by piercing-sucking insects like mealybugs, leafhoppers, scales and aphids is the growth medium for sooty mold fungi. Sooty molds drastically reduce the vigor and beauty of ornamental plants.
I got rid of it but it came back!
There are two kinds of sooty mold. One kind grows on leaves and lasts for the life of the leaf. The second kind grows on stems and twigs of woody plants. During the growing season the fungi produce spores that are blown and splashed to honeydew-coated surfaces like leaves. In wet conditions the spores germinate and sooty mold grows. Hawaii offers environmental conditions that favor the growth of sooty mold which is why it is so common here on the Big island.
How do I get rid of sooty mold?
Because sooty mold is an indication of insect activity, sooty mold is managed by reducing populations of sucking insects like aphids, mealybugs, leafhoppers, and soft scales that excrete honeydew. If you solve your insect problem, the sugar deposits will stop and the sooty mold will eventually go away.
Soap-oil solution by CTAHR
To get rid of honeydew-excreting pests, you can use a simple soap-oil formula recommended by University of Hawaii-CTAHR. It’s great for controlling aphids, mealybugs, whiteflies, scales and citrus black flies. Combine 1 Tablespoon of mild dishwashing liquid such as Ivory, Joy or Dr. Bronner’s (not Dawn) with 1 Cup of vegetable oil such as peanut, safflower, corn, soybean or sunflower oil. Dilute this concentrate with 1 Cup of tap water for every 1-2 Teaspoons. Shake well. Spray plants thoroughly in the morning or late afternoon – especially the undersides of leaves. Spray once a week for two or three weeks. Spray in cooler periods of the day and not in full sun to avoid burning your plants.