What is this black mouldy growth on my citrus leaves?

Citrus sooty mould

Citrus sooty mould is a powdery fungus that grows on branches, leaves, and fruit of citrus trees. Fortunately, it does little harm, and the fruit is edible. However, a severe coating of fungus can block light, adversely affecting plant growth.

Sooty mould is a symptom that your citrus trees have been invaded by harmful insects, usually an infestation of aphids or other sap-sucking pests like scale, mites, whiteflies, or mealybugs.

These pests excrete sticky honeydew that attracts the growth of the ugly black mould.

How to treat Citrus sooty mould

The first step in treatment is to remove the honeydew-producing insects.

Neem oil or Eco-oil are effective ways of controlling pests, usually through repeat spray applications.

How to control Citrus sooty mould

It is also important to keep ants in check, as they are attracted to the honeydew and will protect the honeydew-producing insects from ladybugs and lacewings, their natural predators.

Control ants by wrapping sticky tape around the trunk to prevent ants from climbing up.

Once the pests are controlled, the sooty mould will usually wear away on its own or you can speed up the process by hosing it off with water.