Sooty Mold

Sooty mold may occur on any tree but is most commonly seen on some of the Lilly Pilly species plus numerous ornamental trees and shrubs. These Ascomycetes fungi are generally harmless to plants and can be ignored, except when they are extremely abundant and prevent light from reaching the leaf surface, causing plants to become stressed.

The fungi grow on plant surfaces that have become covered with insect honeydew from aphids, scales, the whitefly, and other insects which suck sap from host plants. The name itself is descriptive, as sooty mold is a black, powdery coating on the leaves of affected trees.

They are most common throughout the warmer months or during vigorous growth periods when large quantities of fresh new growth is present. Once on the tree, the insects continue their infestation until a suitable treatment is carried out.  If sucking insects are not controlled, leaves will drop prematurely, sometimes killing portions of twigs and branches.  Sucking insects also feed on trunks and stems of plants.

Control
Sooty molds can be indirectly controlled by reducing populations of sucking insects that excrete honeydew. Use the appropriate recommended chemicals that control aphids and other sucking insects. Also, a good washing of infested tree’s foliage (if possible) can dilute the honeydew and wash off mold.

Scale insects can actually be contolled with ’white oil". The petroleum base of this product damages their waxy outer covering causing the insect beneath to dehydrate and die. White oil will also lift the sooty mould residue from the leaves. A word of warning - be careful applying white oil in hot, sunny weather as foliage burn can occur.  

Aphids can be controlled with an application of a simple pyrethium, Use a wetting agent like detergent to help chemical to spread and stick to the leaf surface.

Sooty mold on leaves