Pest & Disease Management - Sooty moulds

Sooty moulds can take up levels of management time out of all proportion to the actual damage that they do to plants.

Sooty mould - image: Dove Associates
Sooty mould - image: Dove Associates

The fungi involved are not generally regarded as plant pathogens but live on the sugars in the honeydew excreted by sap-sucking pests. Outdoor and protected crops or plantings can be affected. Only very heavy sooty mould colonisation impacts on plant health by reducing the photosynthetic ability of the leaves.

However, sooty fungal colonies spoil the visual appearance of ornamentals, which can lead to rejection by retail customers. In landscape and amenity situations, plant appearance can be spoiled enough to lead to enquiries or complaints about maintenance standards. Surfaces below affected plants, such as seating or parked vehicles, can also be fouled by sticky, sooty deposits.

Sooty mould can indicate damaging infestations of sap-sucking pests such as aphids, scale insects, leafhoppers and whitefly. So although it might not directly damage the plant, it is worth investigating the cause because feeding insects can result in damage and may be spreading viruses or fungi.

In commercial crops, pest-control treatments will normally prevent sooty moulds. In outdoor amenity plantings, however, this approach is not usually practical unless the pest itself has to be controlled because of the levels of damage that it is doing to the plants.

Scheme design, public information and selection of plant species less attractive to sap-sucking insects can help to reduce the time spent dealing with enquiries and complaints. In interior plantings and public conservatories, careful management of compatible pesticides and biological controls to deal with the pests may be necessary, combined with leaf cleaning and selective pruning.

How to recognise them

Discolouration or superficial deposits of fungal growth can be black, grey or dark brown and soot-like, usually on the upper leaf surfaces.

Sooty mould growth is normally found directly below the feeding sites of sap-sucking pests and may provide a visual warning of pests not otherwise readily noticed such as soft-scale insects. It is always worth fully investigating the outbreaks.

Heavy colonisation, left untreated, may block so much light from the leaves that photosynthesis is reduced enough to weaken plant growth and leave the plant more vulnerable to damage by other pests and fungal diseases. Heavy colonisation may also cause premature leaf-fall.

Biology

Sooty mould is a growth of saprophytic fungi associated with feeding by insect pests such as aphids, adelgids, whitefly, scale insect and mealy bug. These excrete a sticky, sugary fluid (honeydew) - the remains of the sap on which they feed. This accumulates on the leaves below feeding sites, providing a nutrient source for a range of fungi species including Alternaria, Caprodium and Cladosporium.

The sooty deposit is made up of fungal spores and mycelium. These fungi do not penetrate the plant tissues and are often washed off by rain. Sooty mould fungi may also grow on exuding sap or resin associated with wounds. The dark colour of sooty mould deposits absorbs sufficient heat to raise leaf temperatures on warm days.

Most sooty mould fungi are not plant pathogens although research in the past has suggested that some species may directly damage plant tissues by producing enzymes capable of breaking down the pectins and glucans that are structural elements of plant cell walls.

Some trees, notably limes, frequently host non-damaging colonies of aphids so surfaces below them are especially prone to sooty mould growth.

Treatment: cultural control

On commercial crops, maintaining appropriate controls against sap-sucking pests should prevent sooty moulds from causing sufficient cosmetic damage to affect grade-out, prices or customer acceptability.

In landscape and amenity plantings, careful species selection will help to prevent queries and complaints from stakeholders. For example, avoid use of trees such as Tilia spp.

- which are known to carry large aphid populations - in car parks or residential streets because cars parked under them are frequently fouled by honeydew and sooty mould.

For the same reason, avoid placing seating beneath trees that are prone to colonisation by aphids. Some trees, such as catalpa, can produce natural exudations colonised by sooty moulds without the presence of pests.

Include information about the non-pathogenic nature of sooty moulds in local authority literature and web pages relating to parks, open spaces and amenity trees as a means of reducing public enquiries.

In interiorscapes and public conservatory displays, integrated pest-management strategies should keep populations of causal sap-sucking pests in check. However, outbreaks may lead to sooty mould growth, which has to be removed by washing to preserve the amenity value of the plantings or to avoid damage to plant growth.

The leaf washing that should form part of routine maintenance should remove honeydew deposits before sooty mould has a chance to colonise. Regular pruning and thinning to maintain good light levels and air circulation can help to avoid problems on large interior evergreen shrubs and trees.

Treatment: biological control

Sooty mould usually occurs as a result of feeding by sap-sucking pests, so use specific biological controls appropriate to the pest and the crop/planting. In interior landscaping and public conservatory displays, it can be difficult to introduce agents at the right height in the canopy to enable them to find the pests quickly.

Treatment: chemical control

Active ingredient: Prochloraz

FRAC code: 3

Formulation: Octave (Everris)

Action(s): Broad-spectrum protectant and eradicant fungicide. Not compatible with some biological controls.

Active ingredient: Propiconazole

FRAC code: 3

Formulations: Bumper 250 EC* (Makhteshim)

Action(s): A systemic, curative and protectant fungicide with off-label approval for disease control in forest nurseries.

Active ingredient: Tebuconazole

FRAC code: 3

Formulation: Various including Folicur (Bayer CropScience)

Action(s): A systemic fungicide with good action on sooty moulds on wheat.

Fully updated by Dove Associates.

Use plant protection products safely. Always read the label and product information before use.

* EoA required for use in ornamental plant production outdoors and/or under protection.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.


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