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Sooty mould is caused by sap-sucking insects such as aphid, scale or mealy bugs. The insects secrete a sugary substance that goes black with mould over time, hence its name. You may notice there are ants around, as they are also after the sugary substance. Pest oil is registered to control these pests on all ornamentals including natives. Once you control the aphids or scale, the black sooty mould will disappear over time or you can wash off with soapy water.