Sooty mold thrives on honeydew, the secretions of aphids, scale insects and other sap suckers.
In the below picture you can see the white scale insects, ants are present because they are busy "farming" the scale insects for their honeydew secretions and the leaves are also covered in the black, sooty mold which is also thriving on the scale insect honeydew secretions.
One needs to get rid of the sap suckers to get rid of the sooty mold. If not your bonsai may soon take on the appearance below, over and above the damage and spreading of diseases that the sap-sucking insects are inducing by their parasitic activities.
Sometimes when sooty mold presents itself it is the first we may notice of some type of infestation. The quickest solution is to identify the source of the honeydew or in other words which specific insect is secreting it and then dealing with it in the most appropriate way consistent with the level of infestation.
