Powdery mildew on cannabis (Identification and Treatment)

For all gardeners, including cannabis growers, white powdery mildew can be a source of real concern. If you notice a white powdery substance growing on your cannabis plants, it is most likely caused by powdery mildew. You will need to take steps to treat powdery mildew and prevent it from becoming a problem in the future.

This article will introduce and explain all there is to know about powdery mildew on cannabis in an indoor grow room by outlining what powdery mildew (PM) is and how to recognize it. The article will then explain the causes of powder mildew as well as how PM spreads. Finally,we will outline the steps you can take to treat powdery mildew and prevent it in the future.

Cannabis growers should always be on the lookout for white powdery mildew on their crops. If they notice that powder mildew has formed on some of their cannabis crops, they must immediately take necessary action. Allowing powder mildew to remain untreated can lead to economic losses by impacting both cannabis quality and yield.

What is Powdery Mildew?

Whether on a cannabis plant or otherwise, powdery mildew is a fungal disease that often affects all kinds of plants.The disease will show its symptoms through a fine, white powdery substance that builds upon the surface of foliage, typically on stems and leaves. Moreover, PM can most often be seen on the top surfaces of leaves.

Powdery mildew can affect your cannabis plant at any stage of its growth and even affect seedlings and clones. Clones of plants that previously infected with PM are highly likely to get this disease.

As it spreads, it will cause the leaves to curl upwards before eventually killing the plant. Further, if left untreated powdery mildew can devastate an entire indoor grow room rapidly. Importantly, PM has a window of time after inoculation (approx. 4 – 7 days) where it remains invisible to the naked eye.

White powdery mildew thrives in humid environments, especially those with poor ventilation. Humidity levels of around or more than 60% RH are ideal for the inoculation of powder mildew. However, combined with poor ventilation or low air circulation, this recipe for disaster will allow the mildew to settle down.

How to Identify Powdery Mildew

When powdery mildew on plants, the foliage will look like it has white or silvery powder sprinkled on it. This powder, which may resemble flour, usually forms on the upper surface of leaves, sometimes on stems and flower buds.

Further, powdery mildew will cause your cannabis plant’s leave to start curling upwards and wilting. As the fungal disease intensifies, leave can begin twisting and break as the plant becomes weaker. Eventually, the plant will become yellow, then brown, and after that, it will die.

Powdery mildew can potentially affect cannabis plants at any growth stage, although most typically young plants contract the disease. This may be because young plants require high humidity levels, creating the environmental conditions that lead to powder mildew spreading.

What Causes Powdery Mildew?

1.High Humidity

Powdery mildew is a fungal disease, and like many other fungal infections, PM thrives in humid environments. Most ideal for them is the environment in which young crops and seedlings are to be nursed for optimal development (50 – 70% RH). For this reason, powder mildew is most likely to affect young crops.

Humidity levels above 50% can cause powdery mildew to develop. To avoid this from happening, it is important to regularly monitor the humidity levels in your grow room. In addition, cannabis growers should constantly monitor their plants through all the growth stages and look for white powder mildew.

Growers who live in coastal areas have naturally humid environments. You should invest in a dehumidifier to maintain the humidity in your grow room.

2.Poor Air Circulation

While humidity allows powdery mildew to develop, poor air circulation in a grow room can allow it to spread. This is because the air is stagnant for a long time, causing diseases to spread through the air. When air is not moving, fungal diseases like powder mildew can settle down on a plant very easily.

To prevent powdery mildew from becoming a problem, your grow room needs to properly ventilate. While a swinging fan or ventilation system might solve your ventilation problems, it can create an ideal environment for powdery mildew spores to spread from one plant to another.

An air purification system is a much more ideal choice for cannabis grows looking to improve the air circulation of their grow room. Air purifiers can filter out bacteria, spores, and other airborne nuisances and replace the air in the room with fresh air.

3.Insufficient Light

Cannabis plants need plenty of light to develop and grow. If your cannabis plants are not exposed to enough light, this can easily cause powdery mildew to develop. This is because insufficient light causes plants to retain more water inside them. This not only means more humidity but also means your plants can grow weak and anemic.

Further, since fungi tend to thrive in dark and shaded environments, having enough light in your indoor grow room means there is less of a chance for fungal diseases to develop and spread. For those growing cannabis in an indoor grow room, it is therefore essential to invest in good quality lighting and light timers.That can help to ensure your crop receives an adequate amount of light at all times of the day.

4.Excessive Fertilizer

While an adequate amount of fertilizer is necessary for the healthy development of your cannabis crop, excessive fertilization can cause a whole host of problems. One of these issues could be powdery mildew developing on your crop.

An excess of nutrients from crop fertilizer can predispose your cannabis plants to diseases caused by pathogens and fungus. Too many mineral nutrients in the soil can lead to overconsumption by the fertilized plants, which can stunt growth in its way. Further, an excess of nitrogen in the soil can entail what is known as succulent growth, where symptoms of various diseases, like powdery mildew, are exacerbated.

Like many other plants, cannabis requires three key nutrients to develop properly. These are Nitrogen, Phosphorous, and Potassium. Moreover, different NPK ratios are required at different stages of plant growth. If proper nutrient treatment is not followed, this could lead to improper growth and compromise the plant’s ability to go through its development stages properly.

Stunted plant growth is the most common symptom of improper or excessive nutrition. This is because an excess of fertilizer will cause heavy metals to accumulate in the plant, leading to an imbalance in the crop’s chemical composition. Plants may then become weak and will be predisposed to all kinds of diseases, including powdery mildew.

5. Contact between Leaves

As mentioned, high humidity levels in your grow room create an ideal breeding ground for fungal diseases like that, which causes powdery mildew. However, a humid environment is not the only way that plants can experience high humidity.

If leaves of separate plants in your grow room are in contact with one another, this could lead to a higher amount of water retention and increase the humidity levels experienced by plants. Ensuring that there is no contact between the leaves of your cannabis plants is key to preventing powdery mildew from developing in your crop and spreading.

How to Prevent Powdery Mildew

Few steps can be taken to prevent powdery mildew from becoming a problem for your cannabis crop. These have to do with a proper setup for your grow room, as well as regularly monitoring the conditions of your grow room.

1.Climate Control

The first thing to make sure of when preventing powdery mildew from developing is to control the climate of your grow room. This entails controlling the temperature and the humidity of the room; mildew is most likely to develop and become epidemic in warm temperatures, typically between 65 and 85° Fahrenheit.

You can prevent powdery mildew from becoming a problem by keeping the temperature above 85°F. Moreover, if mildew has already become an issue for your crop, keeping a daytime temperature of 90°F in your grow room should kill off most of the fungus. However, these temperatures may kill your cannabis crop, so they should be avoided at all costs.

Temperature and humidity are to be controlled in different ways depending on the growth stage your crop is at. Young Cannabis plants require a temperature of 75 to 80°F, while a slightly lower temperature of 70°F is required for the vegetative stages. For the flowering stage, an ideal temperature is 65 – 75°F.

2.Humidity Control

To prevent powdery mildew from developing on your cannabis crop, the humidity levels in your grow room should be kept under control. As with temperature, different growth stages require different humidity levels, with young plants require the most humid environment of 70% RH.

During the vegetative stages, the humidity levels should be between 40 and 70% and lowered by about 5% each week. Finally, the flowering stages of cannabis require humidity levels of 30% RH. By keeping the humidity of your grow room at the appropriate level for each stage of growth, you can prevent powdery mildew from developing.

3.Air Circulation

To further control the humidity levels of your grow room, you will need to ensure proper air circulation. When air is allowed to circulate properly, excess moisture is constantly removed from the environment, and any air-borne particles, microbes, and fungi do not get the chance to settle down.

You can control air circulation in your grow room by providing proper ventilation and investing in an air purifier.

4.Dehumidifier Usage

A dehumidifier can be installed in your grow room to control the humidity levels of the environment further. If you happen to reside in a naturally humid area, this is all the more important. You can use the required humidity levels at each stage of cannabis growth to calculate the dehumidification demand of your grow room.

When choosing a dehumidifier, certain factors must be kept in mind. For a larger-sized grow room with more plants, a larger dehumidifier would be required.

5.Manage Your Plants

You can prevent powdery mildew from becoming a problem by managing the plants in your grow room properly. This includes ensuring that the room is not too crowded, even at maximum capacity. Moreover, making sure that none of the plants are in contact with one another will lower water retention and help to prevent powdery mildew.

Further, make sure that you are not watering your plants on the leaves and stems. This will increase the amount of humidity your plants’ experience. Further, ensure that you do not overwater your plants as this will have the same effect.

How to Treat Powdery Mildew

If some of your cannabis plants have already contracted the fungal disease that causes powdery mildew, there are a few steps you can take to treat the crop.

How to get rid of powdery mildew?

First of all, you should remove the infected plants from the grow room as soon as possible to prevent the disease from spreading to other plants.

Powdery mildew treatment are use sprays. Powdery mildew cure can also use oil sprays, including neem oil, sesame oil, or fish oil to treat powdery mildew. Another option is a mild spray which is composed of 40% milk and 60% water. These sprays work because of proteins that react with light to form a natural antiseptic.

Although natural sunlight works best for this reaction, it is essential to ensure that your plants are receiving sufficient light when using such sprays in an indoor grow room. Lastly, you can ensure that powder mildew does not spread any more than it already has by wrapping up infected leaves in a plastic bag. This will trap the spores that cause powdery mildew and prevent it from spreading.

Once the infected plants and leaves have been removed from the grow room, you can use a fungicide spray to ensure that the rest of your plants remain safe from this disease.


Powdery mildew is a common source of concern for gardeners and cannabis growers. This fungal disease thrives in warm daytime temperatures, humid environments, and dark/shaded areas. Like other fungal diseases, powdery mildew is spread from one plant to another through spores which can become airborne and settle on plants.

To prevent powdery mildew from developing, you must take certain steps to control your grow room’s climate and air circulation. If you live in a naturally humid area, you may want to consider installing a dehumidifier in your grow room.

Lastly, while powdery mildew can be a real problem, it is not untreatable. By removing infected plants and treating the rest of your plants with oil and milk sprays, or fungicide sprays, powder mildew can be treated, and your plants can be saved from this disease.


If you have interest to know more information about whether the powdery mildew is harmful to humans, you can also get some idea from this awesome artice, “Is Powdery Mildew Harmful to Humans” from Mold Busters, whose mission is to improve quality of life through quality of service!

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