Sooty mould is a fungi which covers plant leaves, stems and twigs in a black sticky substance.
The mould itself does not feed on the plant , however, as it covers the leaf surface, it blocks light and reduces photo- synthesis, essential for plant growth.
In almost all cases, the sooty mould is secondary to infestation of insects that secrete honeydew.
These insects include aphids, scale, mealybug and white flies.
Treating the insects will remove the source of the honeydew and dry up the sooty mould which will eventually fall or wash off the foliage.
The solution to this problem is Conguard® or Pest Guard®, available from Eplants Nursery.