Sooty Mold

Dark, sooty patches on leaves, stems, fruit, or flowers are probably sooty mold.

Sooty mold on citrus leaves (Scot Nelson) Public Domain

Sooty mold refers to several fungi that grow on the honeydew excreted by plant-sucking insects. Honeydew is a delicate way of saying sugary bug poop. Some of the more common insects responsible for sooty mold include the following:

• aphids
• leafhoppers
• mealybugs
• psyllids
• soft scales
• whiteflies

As they feed, these insects excrete much of the sugary sap they eat. The honeydew they leave behind is a petri dish for fungus in the garden. The black, sooty-looking areas you see are most commonly the threadlike mycelium of the Aspergillus niger fungus. Some fungi are plant-specific, while others are less particular.

Sooty mold on eucalyptus leaf (Bidgee) CC BY 3.0

Sooty mold as a symptom

If you see sooty mold in your garden, plant-sucking insects are or recently were present. You can use that information to manage the pests leaving the honeydew behind. Interrupting the lifecycle of these pests will help your garden plants because the honeydew they leave behind interferes with photosynthesis, and many of them carry and spread disease pathogens.

Ant control

Ants will protect, farm, and harvest honeydew-producing insects. You can interrupt this cycle by blocking ants from climbing trees and shrubs. Wrap tree trunks with sticky barriers. Since ants can carry diseases from one plant to another, these barriers are a good idea whether or not you see sooty mold.

Sooty mold control

Sooty mold can get a little out of control in cool temperatures and wet weather. You can use strong streams of water from the hose to displace aphids and other plant-sucking insects. The water will also wash away some sooty mold, but leaves need a wipe down with soapy water to wash off the sugary honeydew that sustains it. Horticultural oil, neem oil, or insecticidal soap can help control the insects that leave the honeydew behind.

You can still eat fruits and vegetables covered with sooty mold. Just wash them off with soap and water first.