One of the most commonly asked questions in our mailbox is about 'black stuff on leaves'. This is normally caused by the the Sooty Mould Fungus (Cladiosporium).
Sooty Mould normally requires an even further down the problems chain in order to make an appearance. For in itself, it does not normally invade plant live - or dead - plant tissue.
Generally it feeds upon the stick substance excreted by all manner of sap sucking insects, known as Honey Dew. Honey Dew, because it is a sweet substance - also favoured by ants.
Tackling Sooty Mould - which in itself is relatively harmless unless there is a severe infestation - is not the answer to the underlying problem.
Sooty mould is present, because there is honeydew for it to feed on. The honeydew is only there because you have failed to spot an insect attack in time. The insects normally at fault, are aphids, scale insects and the like. Basically, sap sucking insects. Sooty mould does NOT grow on the plant tissue - just on the after effects of an insect problem.
An interesting chain of events visible in this image. The sooty mould can be clearly seen on some of the leaves. But prior to that, the eggs of the sap sucking insects can be seen on the flower bud (Top Left corner), as can the sap sucking insects at work, but also many carcases of insects - legs in the air! A closer look to the centre left will reveal a predator of the sap sucking insects. Almost a complete food chain!
Some say that a bad case of sooty mould will stop light getting to the leaf and therefore cause damage. Long before this happens, the insects would have caused damage.
So! No insects = No Honeydew = No Sooty Mould. Treat the cause, not
the after effects which cause the sooty mould.