Mould and bud rot are a serious and common problem that many marijuana growers face every day. It can be very disheartening to check on your healthy crop, only to see the signs of mould setting in, potentially ruining everything. So here is how to spot, prevent and treat mould.


Of all of the types of moulds and infections out there that can attack your cannabis plant, it is Botrytis (bud rot) and Powdery Mildew that are the most common.

Botrytis (Bud Rot):

Botrytis tends to set it at the base of the stem, making it often hard to detect. Usually, the first signs seen by the grower are when the small leaves of the kolas begin to wither, discolour, and dry out. Upon closer inspection, you may notice fluff growing amongst buds and bud sites, usually grey, blue-green, or white in colour. Depending on conditions, this will either cause the bud to dry out completely and crumble when touched (in low humidity), or turn into a decomposing sludge (in high humidity).

When Botrytis reaches advanced stages, you may notice little black specs appear in the mould. These are spores the mould is creating, ready to spread itself to other plants. They can be transported on the slightest breeze. Things are truly dire if you see this. Do not breath these spores in, or allow them anywhere near other plants.

Botrytis can also attack stems, leaves and entire young plants, but this is less common. Just make sure to check your plants regularly!

Mould Cannabis Botrytis Bud Rot

Powdery Mildew:

Powdery Mildew appears as a thin layer of white powdery mould on the leaves of your cannabis plant, before spreading across the entire plant. This inhibits photosynthesis, slowly causing your cannabis to die from lack of energy. Leaves will shrivel, turn yellow, before turning brown and dying. Although hard to spot, Powdery Mildew can sometimes be detected early as small bumps appearing on infected leaves. Like Botrytis, Powdery Mildew produces little black spores in its advanced stages, which can be carried by air movement to other plants.

Both of these diseases can occur indoors and outdoors, as well as infect multiple species of plants besides cannabis, making any plant infection within your grow area or household potentially dangerous to your crop.

Fortunately, both diseases need a very precise set of circumstances to be met before they will infect a plant. This brings us nicely to the next section: Prevention.

Cannabis Powdery Mildew


Without a doubt, prevention is the best way to keep your cannabis safe. It is much easier to take some extra precautions and monitor the state of your growing environment than to try and actually cure an infection.

There are three conditions that need to be met for both Botrytis and Powdery Mildew to take hold. These are:

  • Cool temperatures
  • Stagnant air
  • Wet/humid conditions

So the best way to prevent both diseases is to ensure that no one condition ever persists for long, and certainly not all together.

These are all quite easy to prevent indoors. A cool temperature is anything under 20 degrees Celsius. Ensure you have a decent ventilation system set up to ensure constant air movement, and try to prevent overcrowding of plants that may inhibit air flow. You should also be able to control the humidity of a grow room using ventilation, keeping things in order.

Outdoors is a little trickier. A lack of breeze, cool nights and rain can all stack the odds against you and help mould set in. As such, it is important to choose your grow site with care, ensuring there is a good airflow. Some growers will erect tent-like structure over their bud if they know heavy rain is on the way, and even shake them after a rain spell to shake out any excess water that may be sitting on leaves and buds.

Trust us, when it comes to these diseases prevention is everything.

Humid conditions mould ventilation


As mentioned, treating these diseases is very tough work, but if things have progressed this far, you will have little choice.

First and foremost, consider whether it is worth trying to save an infected plant. As hard as it may be, there is no way to completely cure an infected plant. All you can do is try and keep the disease at bay while healthy sections of the plant grow. However, in doing so, you risk contaminating the rest of the crop for a small amount of bud that is likely going to be sub-par. Treatments will only go so far, and for many, it is better to cut your loss and burn the infected.

Also, once an infection like this sets into a garden or grow room, it is extremely hard to get rid of permanently. Outdoors they will move from plant to plant, as well as reproduce in the soil as mycelia. Indoors, the spores will sit on walls, floors, and pretty much any surface until they are disturbed and moved. It means you run a real risk of infecting future crops as well.


The only real option is to use biological sprays like Serenade disease control spray. This can help treat a botrytis and keep it at bay until harvest, but it won’t get rid of it. Also, move infected plants into a warm room with an extremely low humidity to help prevent spread.

Powdery Mildew:

A bit more can be done with Powdery Mildew. Remove infected leaves and move the plant into a warm, low humidity area. Proceed to spray the plant with safe and specialised fungicides that target the mildew. This should hopefully keep the infection at bay.

It is worth bearing in mind, if you are forced to spray your cannabis with any fungicides, they are going to have an effect on the final bud quality – altering the taste, aroma and quality of smoke. Also, don’t forget that you will be consuming the bud, so do you really want it covered in fungicides or mould? Both can be potentially dangerous – especially if the fungicides are chemical based, or contain copper or sulphur (which many do).

When it comes to Powdery Mildew, there are a few natural gardener’s remedies you can try:

Either add two teaspoons of cider vinegar to 1L of water and spray your plants, or make a mixture of 60% milk and 40% water and spray your plants. Both work to kill Powdery Mildew, but how effective they are is debatable. If you want to avoid fungicides, they are certainly worth a go.

At the end of the day, you have to make your own choice. However, if you ask us, it is always better to remove and destroy infected plants and learn from the experience. It is all about prevention after all!


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