Leaves coated in a blackish film, as if black soot had settled overnight, mark sooty mould (Capnodium oleaginum), a fungus that lives on the sweet honeydew excreted by aphids, whiteflies and scale insects. Nothing works while the honeydew keeps coming, so clear those pests first, then wipe the leaves clean and the foliage gets back to its life-giving function.
This fungal disease is well-known to gardeners, and in itself isn’t particularly dangerous. However, it usually appears in tow of parasite infestations that are much more serious, like aphids or scale insects.
The Latin name of the fungus that develop are Capnodium oleaginum, or Fumago salicina. Spores from these strands develop on sweet honeydew produced by the parasites described above.
The main reason sooty mold develops is the presence of insects and parasites like aphids, whiteflies, and scale insects.
These parasites produce honeydew, which is a sweet and sticky substance. Air-born sooty mold fungus attaches to this honeydew and develops quickly into a black layer that covers leaves.
Although this doesn’t kill the plant, sooty mold does nonetheless reduce photosynthesis when it covers all leaves with a thick black layer. That is when it suffocates leaves that wither and die.
When sooty mold only spots a few leaves, damage is very limited.
Sooty mould always tells the same story, but the amount of black on the plant tells you how far it has gone and what to do next. Use the table below to match what you see with the right response.
| What you see | What it means | What to do |
|---|---|---|
| Sticky, sweet honeydew with aphids, whiteflies or scale insects | The food source the fungus needs to settle on the plant | Deal with the pests first |
| A few leaves lightly spotted with black | Damage is very limited | Wipe the leaves and look for the pests behind it |
| A thick black layer over all the leaves | Photosynthesis drops and the leaves suffocate, wither and die | Treat the insects, then clean the foliage |
| Black deposits on the fruit | The mould is not confined to the foliage | Same approach: pests first, then cleaning |
| A black film still there after the pests have gone | The dark layer keeps blocking light out | Wipe with a damp cloth and lightly soapy water, then rinse |
In every case the insects come first and the cloth comes second.
It isn’t really possible to preventively treat against sooty mold, and when it appears it is immediately recognizable.
There are a few things to do that can reduce the likeliness that this disease would appear.
The only effective treatment against sooty mold is to first treat the insects that are the cause of it.
Here is our advice to avoid or fight them off:
Once the plant is treated and these invasive pests have been removed, clean the leaves so that they may again perform their life-giving function.
Cleaning is easy with a damp cloth or rag dipped in lightly soapy water, which you can then rinse with clear water.
Using organic treatments to treat sooty mold protects the ecosystem that surrounds the plant, most especially bees that come pollinate the flowers.
Sooty mold appears on a great number of plants, such as fruit trees (apple tree, pear tree, olive tree, etc…), heath plants (rhododendron, camellia, hydrangea, etc…), plants in the vegetable patch (tomato, etc…), decorative shrubs (oleander, rose tree) and citrus (orange tree, lemon tree, grapefruit tree, bergamot, etc…)
As you deal with the insects themselves, remember to wipe the sooty mold off the leaves, too. Indeed, even if the pests are gone, that dark layer will still block light out, which isn’t good!
