The presence of sooty mould fungi usually indicates that a plant has become affected by a sap-sucking pest. Sooty moulds do not attack the plant directly, but their growth is unsightly and can reduce plant vigour by preventing photosynthesis. Sooty mold is a secondary problem associated with several insect pests – aphids, mealybugs, scale insects or whiteflies. These insects live by sucking sap from plants, but they don’t digest it completely. The partly digested sap they excrete, called “honeydew,” still contains some sugar. This sticky stuff drips through the plant, coating even parts where there are no insects. When spores of the sooty mold fungus land on it, they begin to grow, using it as a food source.
If you look very carefully at your trees, I think you will find the insect that is feeding the mold. Check growing branch tips, leaf undersides and stems. Aphids are the small, tear-drop-shaped, slow-moving insects we know and hate. Mealybugs are white, fluffy and immobile. Whiteflies are tiny white insects that flutter up when disturbed, though their immature stages are flat, transparent discs that don’t move around. Scale insects are very common and less easily detected than the others. They look like small oval bumps on the plant stems, leaves or fruit.
You may see the following symptoms:
- Black or dark brown, superficial fungal growth on the aerial parts of plants, particularly the upper leaf surfaces
- The amount of growth can vary from a fine soot-like or powdery deposit, to a thick sheet of growth that may crack or peel away from the leaf surface during dry conditions
- The growth can sometimes be washed away, leaving a healthy-looking leaf surface beneath
- Sap-sucking pests such as aphids, scale insects, mealybugs or whiteflies can often be found on the plant, above the point where the sooty mould is growing
- In some cases these insects may occur on plants that overhang those affected with sooty mould
- Ants may also be seen in association with the sap-sucking pests
- Leaves, stems, fruit, etc. where the sooty moulds are growing are contaminated with sticky honeydew
- Wiping or sponging affected leaves and other plant parts with water is sometimes enough to remove the sooty mould growth. However, if the source of the problem is not dealt with then the growth will develop again
- Avoid adding strong soaps or detergents to the water as these can scorch foliage
- Lukewarm water can be more effective
- Heavy coverage can take some weeks or months to be washed away by rain even if insects removed
Apply potassium bicarbonate 90% diluted in 1:100 at 10-15 day intervals. Apply to new growth when infections occur or if conditions favour disease. When disease pressure is high or infection is severe apply at heavier rates and shorten the spray interval to 7-10 days until disease is controlled. Click “here” for link.
Chemical control of the sooty mould growth itself is not required. However, control of the sap-sucking pest responsible for the honeydew on which the mould is growing may involve the use of pesticides.
Sooty moulds are surface contaminants – they do not attack the plant directly. They require a nutrient source on which to grow, and this is most commonly the honeydew excreted by a number of sap-sucking pests (e.g. aphids, mealybugs, scale insects, whiteflies). Droplets of honeydew are shed by these pests and fall onto surfaces below where they are feeding. This is frequently the upper surface of leaves, but can also be stems, branches, fruit, etc., and also any other objects situated below the infestation.
Honeydew contains high levels of sugars and a range of other nutrients. The sooty mould fungi use these for growth. Sooty mould growth is most prevalent where air circulation is poor and humidity high, providing periods of extended wetness (although heavy rain may sometimes wash the growth from the leaf surface).
Occasionally, sooty mould growth develops on sugary, sticky exudates produced by the leaves of the plant itself. Certain plants (e.g. some Cistus species) are more likely to produce such exudates.