August, 15
by RT

Preventing Cannabis Mold

It's no secret that mold on cannabis is a bummer for any grower. You may make it through half or all of the stages of cannabis growth before a plant or an entire crop becomes infected.

Preventing Cannabis Mold

While there are many different claims about saving a plant that is suffering from mold or using it anyway, it's important to note how dangerous different types of mold are and how they can negatively affect the person smoking or ingesting it. Do your research.

Instead of going into the he-say-she-say of curing and/or smoking moldy cannabis, we are going to cover preventative measures and the types of molds to watch out for so that growers and smokers can avoid the risk altogether.

First things first, there are 3 times you are most likely to see mold on your cannabis:

  1. During the vegetative stage, when your plants are big and leafy, they are growing in an environment that is hot and humid without much airflow. The more vegetation plants have the more they are releasing water into the air. The smaller the growing space is the less amount of airflow is being circulated. This develops pockets of stagnant air and is very common in smaller growing environments such as tents. These conditions are ideal for the growth of white powdery mildew which may appear as small circles of white dust on the tops of leaves and stems.
  2. In the last few weeks of flowering, buds become more sensitive to mold and the larger and denser the colas are, the easier it is for them to become infected. Mold can appear in a variety of colors and in this stage is generally referred to as bud rot or Botrytis. With bud rot, the leaves around the cola will turn yellow or become loose and fall out and the bud itself will become discolored. If you notice a visual difference in one bud but not the others, it's important to evaluate your plant immediately.
  3. Let's say you've made it through all the stages of growth and it's finally time for drying and curing. The methods that you use will not only have an effect on the appearance and quality of your bud but can also prevent or generate mold. It's critical to not skip any steps and to check on your cannabis regularly. It's recommended that if you store cannabis in jars, you open the lids once a day to release toxins and let in fresh air.

Now that you know when to look, let's get into the different types of mold, what you’re looking for and how to prevent them. 


Signs of Illness

 Botrytis, also known as grey mold or bud rot, is a pathogenic fungus and one of the most common fungi found on marijuana plants.

Botrytis is easy to detect as the color will change from brown to gray and the texture will become dry.

Botrytis can affect the roots, stems, leaves, and buds of a plant and spreads fast with the ability to kill off a plant in just days.

Cure / Prevention

If you notice fluffy white growth in the middle or on the sides of your buds you have detected bud rot in the early infection stages.

Typically spread by wind or rainwater, Botrytis will germinate plants through an injury.

If your plant has been exposed to spores, the bud rot cycle will not begin if you provide a warm, dry and airy environment.

  • Humidity - control humidity levels by reducing it to less than 50%
  • Propolis or bee glue - Can be purchased through online grow shops or retail stores and used immediately
  • Remove & dispose of the infected portions of your plant (create a safety margin by cutting off a bit of the healthy part)
  • Ventilate the growing area by using a HEPA air filtration unit (MERV15 or higher is recommended)
  • Horsetail - 1 ounce of powder makes a gallon of ready-to-spray solution
  • Essential oil extract - Thyme, rosemary, eucalyptus, black caraway & lemongrass can be used as diffusers to prevent bud rot from attacking your plants.
  • Silica - Can be used in soil to help strengthen stems, acts as an alkaline adjustor and can be easily administered through a high-silica fertilizer

Powdery Mildew

Signs of Illness

In the beginning stages of powdery mildew or PM, you may notice small circles of white dust on the tops of leaves and stems. As the fungus progresses it will become denser, affecting the leaves, buds, and stem by covering them in a thick white mildew.

Powdery mildew generally affects older leaves first and may cause some to break or become distorted.


Powdery mildew is not the most dangerous fungus, but when left untreated will enter buds preventing safe intake. To stop this fungus from attacking your plants you can take the following precautions:

  • Horsetail - 1 ounce of powder makes a gallon of ready-to-spray solution
  • Propolis or bee glue - Can be purchased through online grow shops or retail stores and used immediately
  • Capture & remove - A self-supported, MERV15 or higher HEPA air cleaner will actively remove sub-microns preventing Powdery Mildew(
  • Trichoderma Harzianum - Used in the soil to effectively prevent powdery mildew


Signs of Illness 

Fusarium is filamentous fungi or parasites that lives within soil and feed on plants, infecting and ultimately killing them. Depending upon the strain of marijuana, Fusarium may cause wilt while in others root rot. Fusarium can remain dormant in soil for years and is spread through water movement or after coming into contact with garden tools.

If your plant is suffering from Fusarium Wilt you will notice dark spots on the lower leaves which will quickly turn the leaves a yellowish- brown color. Leaves will wilt but not fall off and your stems will droop.

Fusarium Root Rot will turn the roots of your

plant red spreading up through the stem causing it to swell, break open, wilt and collapse. 

Cure / Prevention

As soon as you are aware of Fusarium, you must remove the infected parts of the plant. If you are growing outdoors you can relocate to a new outdoor area or move your plants inside. If you are dealing with a Fusarium attack you will need to dispose of everything including the seeds. To prevent Fusarium you can try these treatments: 

  • Crop rotation - Growing marijuana in one location will cause the soil to become worn making it more adapt to infection
  • Compost - Use properly aged compost for soil
  • Clean & sanitize garden tools after use to prevent contamination
  • Trichoderma Harzianum - Used in the soil to effectively prevent powdery mildew Prevent fungi - With proper air filtration (MERV15 or MERV17) you can guarantee the elimination of Fusarium in your grow room.

Phytophthora Infestans

Signs of Illness

Phytophthora infestans show similar signs to powdery mildew, but the fungus can be found on both the tops and undersides of leaves. The primary signs of infection include small translucent or yellow stains which may or may not be followed by white dust on the underside of the leaves.

Cure / Prevention

If you identify Phytophthora Infestans it’s critical that you improve ventilation, remove the affected areas of the plant and make sure there is a proficient amount of space between your plants.

  • Bordo Mix - An organic fungicide mix made up of copper, sulfate & slaked lime and used as a spray
  • Grow room filtration system - Ensure the cleanest grow room air with a highly effective air cleaning system. Filters recommended: MERV15 (95%) or MERV17 (HEPA).

Aspergillus, Penicillium and Rhizopus Mold & Micro Organisms

Signs of Illness

Mold and microorganisms impact both indoor and outdoor growers, vary in color and appearance and have different effects on humans and animals. It's best practice that growers learn, understand and examine plants throughout all growing stages to protect plants, customers, and patients. 

  • Aspergillus - Common mold found both in and outside, generally dark green-black spores that create an infection in the lungs after being inhaled and can be found in soil or buds
  • Penicillium - Predominantly indoor mold, typically light blue-green spores that can ruin crops and infect and harm both humans and animals
  • Rhizopus - Produces grey-black spores and may infect humans under certain conditions such as when an individual’s immune system is compromised 


If you have identified Aspergillus, Penicillium or Rhizopus on your plants, it's important to understand that the mold you see is not your biggest concern. Aspergillus, Penicillium and Rhizopus fungus spores drift into the air landing on different areas of the plant and can survive basically anything. If you notice any of these molds your cannabis must be thrown away. However, by implementing proper precautions early you can avoid the dangers of these microorganisms. 

  • Hygrometer - Measures the humidity of the air
  • Fresh air - Ventilating your grow room with a dynamic filtration system will establish the highest quality of air while preventing mold and microorganisms from contaminating your plants 
  • Silica gel packets - Keep moisture levels down

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