The presence of sooty mould fungi usually indicates that a plant has become affected by a sap-sucking insect. Sooty moulds do not attack the plant directly, but their growth is unsightly and can reduce plant vigour by preventing photosynthesis.
Scientific name Many different species of fungi
Plants affected Any. Growth may also occur on other objects such as garden furniture, paths, cars, etc.
Main symptoms Superficial, dark brown or black fungal growth
Caused by Fungi, growing on insect honeydew or plant exudates
Timing During periods of plant growth and insect activity; may be year-round in conservatories and glasshouses
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.
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 insects 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 insects
- 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 the insect producing the honeydew has been controlled
Chemical control of the sooty mould growth itself is not required. However, control of the sap-sucking insect responsible for the honeydew on which the mould is growing may, depending on the gardener's approach, 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 insects (e.g. aphids, mealybugs, scale insects, whiteflies). Droplets of honeydew are shed by these insects 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.
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.
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