How to recognise it
Black or dark brown deposits on upper leaf surfaces and other parts of the plant.
The best way to identify an infection of sooty mould is to search for the pest causing it.
Why it’s a problem
The sooty mould does not directly harm the plant, but it is unsightly (particularly on fruits) and if it covers large areas of the plant it can reduce the plant’s ability to photosynthesise. This reduction in photosynthesis can cause leaf fall and a general weakening of the plant.
Where you are likely to find it
Due to the wide range of pests which can produce the honeydew which attracts sooty mould, there is equally a wide range of plants which can be affected. It can particularly affect plants in glasshouses and houseplants which don’t benefit from regular rainfall to wash the honeydew and sooty mould off the plant.
How to prevent it
Sooty mould is caused by sap feeing insects (such aphids, adelgids, whiteflies, scale insects or mealybugs) which secrete a sticky, sugary liquid called honeydew. Droplets of the honeydew accumulate on plant surfaces below the pest’s feeding area and attract fungi (such as Cladosporium spp.) which produce the sooty deposits. Therefore controlling the pests is ultimately the way to prevent sooty mould from forming.
How to get rid of it
Affected leaves can be sprayed or sponged with water to remove thick deposits. Once cleaned, affected fruits should still be edible since the mould does not penetrate below the surface.
The pest in question should be dealt with to prevent recurrence.
Is it good for anything?!
It can provide a useful sign of pest infection which can be more damaging to the plant than the sooty mould itself.
Other useful information
Cars and garden furniture can also be affected by sooty mould, if left under trees or shrubs where sap feeding pests are present. They should be moved or washed frequently to prevent damage to the paintwork.