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Sooty mold, techniques and organic treatments to avoid it

Soothy mold symptoms cause treatment

When leaves are covered with a blackish substance, similar to black soot that would have collected overnight, it is a sign of sooty mold.

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.

Sooty mold symptoms, black powder on leaves

Spots of black on leavesThe 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.

  • Note that it might also appear on fruits.

Preventive treatment against sooty mold

Sooty mold is a mold that feeds off honeydew, which aphids excreteIt 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.

  • Ensure your plant has sufficient nutrients and is well watered. A plant weakened by poor soil or lack of water is a vulnerable plant.
  • Increase the diversity of plants and flowers in your garden, it is the best way to protect them naturally from their predators.

Effective treatments against sooty mold

The only effective treatment against sooty mold is to first treat the insects that are the cause of it.

Sooty mold treatmentHere 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.

Plants impacted by sooty mold

Scale and sooty mold on a mock-orange leafSooty 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…)


Smart tip about sooty mold

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!

Images: Public Domain: Scot Nelson, U.S. Department of Agriculture
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  • susan S. wrote on 17 January 2022 at 2 h 02 min

    The successful way to get rid of sooty mold and black spots on camellias is to defoliate the tree or bush and the next growth cycle will be beautiful and free of problems.

    • Gaspard wrote on 17 January 2022 at 8 h 37 min

      Hi Susan, it’s true that defoliation will get rid of all infected leaves. In the case of black spot, it’ll also reduce the risk of having new spots form: the spots are actually where new spores form that will then spread and infect other parts of the plant.

      However, it’s a bit different for sooty mold: the black dust-like coating that’s on leaves is a fungus, but it doesn’t feed on the leaves: it feeds on honeydew that’s left behind by insects (usually aphids). So removing the leaves won’t really heal the plant if there are still aphids around. It’s important to get rid of those first. If you don’t, any new growth will be overrun by aphids, which will produce honeydew again, and sooty mold will develop again on new leaves. To make things worse, the defoliation will have weakened the tree and created lots of new wounds where leaves were torn out. So overall, the tree will be worse off.

      To sum it up:

      • for black spot, defoliation is a good option (again just once a year max to avoid weakening the plant).
      • for sooty mold, defoliation will probably weaken the plant without solving the problem: getting rid of insects and pests instead should do the trick

      Once you’ve dealt with the insects and there aren’t any left to spread honeydew, though, you can defoliate so that the shrub produces a new set of “clean” leaves that are much nicer to look at!