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Powdery Mildew On Cannabis – The Ultimate Solution

Powdery mildew on cannabis is extremely common and can destroy entire grow cycles. Traditional foliar sprays aren’t suitable for cannabis cultivation, so the only effective solution to prevent powdery mildew is dehumidification.

Powdery mildew is a very common mold, found around the world. It affects a wide range of plants, including tomatoes, peppers, cucumbers, and melons. But it’s especially difficult to deal with powdery mildew on cannabis.

Powdery mildew spores are present everywhere, indoors, and outdoors, making them extremely difficult to avoid, even in isolated grow rooms.

Effects and Symptoms of Powdery Mildew on Cannabis

As in all crops, powdery mildew slows down the growth of cannabis plants. When gone untreated, it can cause plants to shrivel and die.

Powdery mildew gets its name from the powdery coating that covers infected plants, usually on the upper side of the leaves.

Symptoms of powdery mildew vary, but usually first appear on older leaves, lower on the plant. Some of the early symptoms you should look for include loss of color and bright green or yellow spots.

Powdery mildew spores usually develop on leaves. But you may also find them under the leaves, on the stems and on petioles.

Dehumidification Is the Ultimate Solution to Powdery Mildew on Cannabis

Preventing powdery mildew in cannabis greenhouses and grow rooms is tricky. Unlike other molds and mildews, it doesn’t require the presence of water to develop. Rather, all it takes is a brief period of high humidity, for spores to begin to germinate.

The most reliable and efficient solution to prevent powdery mildew is to maintain a steady relative humidity level. That means using dehumidifiers to extract moisture from the air, as well as avoiding temperature fluctuations.

Cannabis requires a temperature range of roughly 20-30°C. Within this range, it grows fastest, largest, and healthiest, producing the most buds, at the highest quality. However, the more temperatures fluctuate, even within this range, the more likely it is to experience a powdery mildew outbreak.

Temperature fluctuations inevitably lead to fluctuations in humidity, due to the nature of relative humidity. Warm air can hold more moisture than cooler air. So, at lower temperatures, the same amount of water vapor will lead to higher relative humidity.

Any temperature dip will cause short relative humidity spikes. They may not be felt, as relative humidity doesn’t always rise high enough to cause dew point condensation. But it may be enough for powdery mildew spore germination to occur.

Luckily, temperature fluctuations occur at set times during the day, so growers can easily avoid them. Temperatures usually drop at sunset and spike at sunrise. The same is true when grow lights are turned off or on.

Using DryGair to Prevent Powdery Mildew on Cannabis

To ensure powdery mildew prevention, growers should turn to dehumidification solutions, such as DryGair.

For best results, DryGair should run as necessary. Meaning whenever it’s not possible or practical to release moisture through ventilation.

In indoor grow rooms, it’s recommended to operate DryGair to maintain a steady relative humidity setpoint at all times. This allows the HVAC, or other heating and cooling systems to efficiently maintain a steady temperature.

Using Air Circulation to Enhance Dehumidification

Dehumidification alone can keep relative humidity at bay in greenhouses and indoor grow rooms.

However, as cannabis buds are dense in their nature, moisture can get trapped. So, even if relative humidity is generally kept low, it could still be high inside the bud, and around the plant.

To disperse these pockets of humid air, known as microclimates, growers use fans. DryGair dehumidifiers take things one step further, providing a patented air circulation mechanism.

Traditional fans push air in a linear direction. This creates fluctuations in conditions between different areas. While plants close to the fans may avoid microclimates, those further away may suffer.

Furthermore, the relative humidity and temperatures plants experience may be completely different, creating differences in bud development and quality.

However, air circulation creates uniform conditions everywhere in the grow room or greenhouse.

Whether circulating air, or using directional fans, air flow helps prevent powdery mildew and other molds from developing.

Additional Preventative Measures for Powdery Mildew in Cannabis

Cannabis is highly regulated, so growers can’t use traditional fungicides, being too toxic. But there are several alternative methods to prevent or kill powdery mildew.

It’s important to note that prevention is always better than post-outbreak treatment, abiding with integrated pest management principles.

Preventing powdery mildew isn’t only more beneficial and cost-effective, it’s also easier. It’s extremely difficult to stop spores from spreading and developing in the growing facility, once plants are infected. Even if you do stop the spread, it will inevitably lead to some yield loss.

Growers can utilize a mixture of the following methods to help prevent or eliminate powdery mildew:

Prune and Space Out Plants

The simplest method on this list is just an extension of climate control. If dehumidification and air circulation fail to prevent mold outbreaks, it’s likely due to persisting microclimates.

One way to further avoid microclimates, is by creating more space between plants. Placing them further away from each other, or pruning leaves, when possible, can have a tremendous effect. Humid microclimates are often to blame when it comes to powdery mildew outbreaks. So preventing them is the key.

It’s important to note that pruning isn’t only useful in creating space. It’s also critical when powdery mildew symptoms begin to appear. You should always prune infected leaves, and completely remove plants with major infections, to decrease the chances of spreading.

UVC Lights

UVC lighting is a relatively new method of preventing powdery mildew in cannabis and other crops. It refers to a specific spectrum of ultraviolet light, which is non-visible. This type of radiation can alter or damage the DNA of microorganisms.

So, growers can use UVC lights, to an extent, to damage powdery mildew spores, preventing them from replicating and developing.

UVC radiation may sound like is a magic solution to eradicate powdery mildew, but it’s not that simple. While UVC light inhibits spores from developing, it’s not as effective once the mildew takes hold.

Additionally, to be effective against powdery mildew, growers need to apply UVC radiation at a very specific dose.

If lighting intensity or duration aren’t enough, spores won’t be affected. On the other hand, too much UVC radiation may burn the cannabis plants. Getting the right intensity and duration of UVC radiation is complicated, and growers often don’t achieve the desired results.

All-in-all, growers may use UVC lights to assist in powdery mildew prevention. But they might not be effective as a sole solution.

Air Filters

Air treatment equipment, such as HVAC, can easily spread powdery mildew spores throughout the entire facility. However, certain air filters may be able to achieve the opposite, capturing powdery mildew spores and preventing them from reentering the space.

Some air filters, with a high MERV rating (minimum efficiency reporting value), are reportedly capable of capturing powdery mildew spores.

The downside of relying on air filters to prevent powdery mildew is that they aren’t always as effective as needed. They also require constant maintenance and filter replacement.

It’s important to note that maintaining a spore-less environment is close to impossible. Fungal spores are omnipresent and may arrive from anywhere, including from vents, on equipment, or with workers.

Preventing the conditions spores require to develop into mold is much more effective and easily achievable.

Potassium Bicarbonate or Baking Soda

Powdery mildew requires a certain pH level on the leaf surface. So, raising or lowering its acidity level can disrupt powdery mildew development.

using a pH changing agent, such as potassium bicarbonate, or sodium bicarbonate (also known as baking soda), may help prevent powdery mildew.

Applying bicarbonates raises pH levels, albeit temporarily. So, in order to prevent powdery mildew, growers must routinely apply the chemical preemptively.

Potassium bicarbonate and sodium bicarbonate are approved for organic use. However, they may have a negative effect on cannabis, especially during flowering.

Many cannabis growers avoid these materials, and prefer to prevent molds through climate control, rather than applying chemicals.

Neem Oil

Another natural fungicide and pesticide that can eliminate powdery mildew spores is neem oil.

Neem oil is a natural extract of the India-native neem tree. It’s extremely common among organic farmers and even home-gardeners, treating various fungal diseases and preventing pest infestation.

To prepare a usable neem oil mixture, simply mix 3 tablespoons of neem oil in a gallon of water. Apply the neem oil mixture on leaves, from both sides, once every week or two, to prevent powdery mildew development.

Neem oil, while somewhat effective, requires a lot of manual labor in order to effectively prevent outbreaks. So even though it’s very popular among small-scale grows, most commercial cannabis growers prefer more efficient solutions.


Biofungicides are the last resort and the most toxic solution on this list. In fact, their use may not be permitted in every commercial cannabis grow.

Biofungicides are a group of products that trigger the plant’s defense mechanism, producing a biochemical that fights off disease.

To successfully prevent powdery mildew outbreaks, it’s necessary to apply these materials once every week or two.

Cannabis growers rarely use biofungicides, due to their toxicity and the strict standards surrounding cannabis production.

Check out our full guide to humidity related diseases to learn more about powdery mildew and other common greenhouse ailments.

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