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What Is Sooty Mold And How Does It Affect Cannabis?

Created by
JoshuaHolt JoshuaHolt
Added 22 January 2021

Soot Mold On Cannabis

Black powder on your cannabis plants is definitely not normal and generally means you have another problem elsewhere. Fortunately, sooty mold itself is not too difficult to get rid of and it can give us a good indication our plants need some serious attention. This article guides you through the diagnosis, treatment and prevention of sooty mold so you can get your girls back in top shape.

What Is Sooty Mold?

What Is Sooty Mold

Sooty mold, also known as black mold, is a type of fungal disease that affects many types of deciduous trees and shrubs, including fruits and vegetables. It appears as a black, charcoal like powder which coats affected areas of the plant such as leaves, stems, flowers or fruits. As the name suggests, it is very like soot.

Sooty mold is a term that describes the substance caused by different species of Ascomycota fungi that are known to infect plants and appear because of various insects that excrete a sticky, sweet substance known as honeydew.

The mold also survives off sugars released by the plant through wounds, such as pierced leaves from pest bites. 

How Does Sooty Mold Affect Cannabis?

How Does Sooty Mold Affect Cannabis

Sooty mold affects cannabis in a number of ways and it does not always appear on the same part of the plant. However, sooty mold is most commonly found growing on the leaves, where other problematic pests can generally be found.

Most often, sooty mold develops as a result of a problem elsewhere, such as another pest, or from damage to the plant itself. It doesn't do a lot of damage directly to cannabis, but it already indicates there is another issue at hand.

Sooty Mold Is Often Caused By Honeydew

If left unchecked, sooty mold can ultimately cover the leaves and block them from receiving light, which stunts photosynthesis and therefore your plant's ability to function and effectively use nutrients.

Symptoms of Sooty Mold:

  • Black, charcoal spots growing on leaves or stems
  • Withered/discoloured leaves
  • Slow plant growth from reduced photosynthesis
  • Pests that produce honeydew such as aphids, whiteflies or mealy bugs

Like other fungus, sooty mold survives by consuming plant matter to multiply and spread its spores to other nearby plants. This eventually drains a cannabis plant of its nutrients and creates a bacteria filled environment that invites other pests or molds to settle in too.

The Causes Of Sooty Mold

Aphid Infestation

The appearance of sooty mold comes down to a few factors. If you notice sooty mold growing on your cannabis plants, you'll need to consider the possibility of pests, environmental imbalance, plant damage and rot. Improper conditions in combination with bad hygienic practices can always be a reason for mold to start colonising.

  • Honeydew from other pests such as aphids, whiteflies or mealy bugs.
  • High or low humidity, especially during the night.
  • Cuts on plant tissues or broken stems.

Sooty mold will normally appear due to a combination of these factors, so do not hesitate and figure out what is causing it as soon as possible.

Treating Sooty Mold

Cannabis Pests

Like we mentioned before, you may have a more serious pest problem on your hands that needs thorough treatment to get rid of before being sure sooty mold won't appear again.

To effectively and quickly treat sooty mold, removing the affected areas and discarding them does the trick. However, depending on how much foliage your cannabis plants have (as well as the severity of the infestation and stage of growth), removing foliage may or may not be an option.

Consider your options before removing too much foliage, which can stress our plants even further and also spread spores unnecessarily if we are not careful. On that note, you may want to treat the plant in a separate space away from other plants to avoid potentially infecting other plants.

Pruning Sooty Mold

Treatments for sooty molds:

Remove affected foliage - Pruning can help to eradicate a large portion of the colony, and likely some of the pests that caused it. Sterilise all equipment and discard in a warm bucket of soapy water to kill remaining spores.

Wipe it off - After pruning, use a warm, damp, soapy cloth to clean off any remaining mold.

Foliar spray - Many pests can be treated using foliar sprays. Whether organic or chemical (preferably choose organic but you may need something stronger depending on the pest problem and severity), be sure to avoid spraying on the flowers in the last 3 weeks before harvest.

Pest treatment - If the discovery of sooty mold has lead you to find a pest, look into how to deal with that particular pest. You may want to introduce predators or apply sprays, but preferably not both at the same time. Add predators once you're sure there is no residue left from any foliar sprays.

For pest management, feel free to use our guide on identifying common pests if you are wondering which pest may have found a way into your cannabis garden.

Tip: Wear a facemask and gloves when dealing with sooty mold, and be gentle if removing and discarding infected parts.

Tips For Preventing Sooty Mold

Foliar Spray During Vegetation

Sooty mold is relatively simple to deal with in its early stages, however you may need to put in some dedicated time into preventing altogether.

Prevention is the best way to solve not just one problem, but a domino effect of poor health symptoms and eventually low yields, bad quality weed, or even the death of the plant.

  • Be cautious when handling plants infected with molds, as breathing the spores can be harmful for the respiratory system. It is also possible to be allergic.
  • Install an air purifier to remove fungal spores from the air.
  • Provide enough ventilation and add oscillating fans to prevent stagnant, humid air.
  • Keep humidity and temperature within the proper ranges. Try not to let it exceed 26-27°C during the day and avoid it dropping below 15-16°C at night.
  • Regularly check for pests and do not let them get out of hand if you do find any.
  • Use routine foliar spray during vegetation, such as neem oil or potassium soap.

Pest prevention by YouCannaBlowMe from GrowDiaries.

Finally, always keep your grow room and plants as clean as possible. That means it's important to regularly clear up decaying or dead leaves, wipe up any spillages or grow medium, filter your water, replenish the air, and all those other things that come with maintaining a healthy environment in the grow space.


Although you will hopefully have dealt with the initial problem before sooty mold becomes a serious challenge, it may be that you don't notice anything wrong with your plants until sooty mold shows itself. The best thing to do is monitor your plants daily so you can spot any molds or pests quickly if they do ever get cosy in your grow room.

If you found this article useful or have any tips regarding the treatment of sooty mold, please feel free to leave a comment down below for fellow GrowDiaries users!

External References

Sooty Mold. - Nelson, Scot. (2021). 

Contamination and relevance of sooty mold prevalence in urban greeneries. - Baležentienė, Ligita & V., Snieškienė & A., Stankevičienė. (2015). 

The Sooty Mold Ascomycete Genus Limacinula. Mycologia. - Reynolds, Don. (1971).

This article was updated April 2021.