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.
What are sooty moulds?
Sooty moulds consist of a large number of different fungi producing dark growth and spores. Commonly found within sooty mould growth are species of Cladosporium and Alternaria, but there are many others. They grow on sugar-rich honeydew produced by sap-sucking insects as a result of their feeding activities, or occasionally on sugary exudates produced by the plant itself.
- 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
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.
See our advice on controlling aphids, scale insects, mealybugs or whiteflies.
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.