Problem Identifier / How to Treat And Cure Powdery Mildew on Cannabis

If you are experiencing powdery mildew on non-cannabis plants, please click here for more information.

What is Powdery Mildew on Cannabis?

Powdery Mildew Cannabis
Early signs of powdery mildew on a cannabis plant. PHOTO CREDIT Whitney Cranshaw, Colorado State University,

Powdery mildew is a parasitic fungus also called white mold and Oidium. It is often referred to as just ‘PM’. Powdery mildew fungi are referred to as Ascomycetes and are in the Erysiphales order. Due to the parasitic nature of powdery mildew, live tissue is required for growth and reproduction.

The fungus attacks the aerial sections of your plants. If not treated early, powdery mildew will continue attacking your crops by damaging the leaves, petioles, stems and buds. If left to progress, your plants will be unsuitable for smoking. Powdery mildew is a common fungus for cannabis crops.

The fungal spores can lie dormant in the environment (indoor or outdoor) for a long time due to resiliency. This will make future cannabis crops/plants are susceptible to powdery mildew if action is not taken to eradicate the issue.

What Does Powdery Mildew Look Like?

  • Powdery and fuzzy flour-like white patches on your plant leaves
  • Distorted leaves
  • Discolored plant tissues
  • Newer growth is affected

Once PM has infected your plants, you will see a white dusting like flour. This generally begins with powdery, circular white spots appearing on the stem and leaves. In most cases, powdery mildew appears on the upper sections of the leaves but can sometimes be seen on the undersides. Younger foliage is most likely to receive damage, but it can also colonize mature tissues. The leaves start to yellow before drying out.

In some instances, leaves can become disfigured, break off or twist. Eventually, most of your leaves will be covered with white areas of powdery mildew. You will see a disfigurement of the leaves, growing tips and buds. The symptoms of PM generally appear toward the end of the growing season.

How Does Cannabis Get Powdery Mildew?

Environmental factors including high humidity, poor and unfiltered airflow and introducing infected clones/plants are the main cause of powdery mildew.

Take care of untrained plants with bushy leaves because of consistent new growth. If plants can touch, this can create airflow dead spots where PM can thrive.

Always be sure to allow enough space between plants. You can significantly decrease the areas where PM settles by defoliating some of your fan leaves. Choosing the leaves shaded from light also frees up plant energy to increase your yield.

How to Treat Powdery Mildew

How to Kill Powdery Mildew on Cannabis During Flowering

You can treat PM during flower up to two weeks before harvesting using Trifecta Crop Control. Apply 2oz per gallon every 72 hours until the infestation is under control.

Treating Powdery Mildew on Cannabis Indoors

Treat Powdery Mildew Cannabis
A more progressive case of powdery mildew.

Indoor PM can be treated with an application of Trifecta Crop Control Super Concentrate every 72 hours. Use two ounces per gallon until the powdery mildew is under control. Then apply one ounce per gallon every week for prevention. Make certain your plants receive good airflow and ventilation.

Once the infestation is controlled, you will need to sterilize your grow room thoroughly to kill any mold spores that may have survived. Natural options for doing this include:

  • Sodium bicarbonate and water solution as it is a much higher pH than powdery mildew can withstand.
  • Ozone generator. Be sure to ventilate before re-entering your room.

How to Treat Powdery Mildew on Cannabis Outdoors

Trifecta Crop Control is highly effective at treating powdery mildew in both indoor and outdoor environments. Apply 2oz of Trifecta Crop Control Super Concentrate every 72 hours until the powdery mildew is under control. For preventative maintenance, use 1oz weekly per gallon. To ensure an effective application, you must consider both the weather conditions and relative humidity.

If the damage is severe, remove all infected foliage including stems and leaves. Throw everything into the trash or burn when possible. Do not use compost on any of your infected plants because the wind will spread the PM resulting in compromised compost materials.

How to Prevent Powdery Mildew on Cannabis

How to Prevent Powdery Mildew on Cannabis Indoors

Apply Trifecta Crop Control as a preventative

We cannot stress the importance of preventative! Powdery mildew (as well as many other pests, mold or mildew) will never become an issue if you apply Trifecta Crop Control Super Concentrate as a preventative at 1oz per gallon, once per week.

Control your humidity

The most important thing you can do. Higher humidity is sometime required so PM will only usually become an issue if it is combined with low or no airflow.

Powdery Mildew On Cannabis
Early signs of powdery mildew on cannabis. PHOTO CREDIT Whitney Cranshaw, Colorado State University,
Ensure cleanliness of your growing environment!

Additional preventative measures are necessary for controlling the environment of your grow space. Cleanliness is extremely important! A good option is creating a decontamination area in your facility for all personnel entering the building. This will decrease the risk of infection from outside.

This area should be reserved for changing from civilian clothing into appropriate work attire. This action will decrease infection risks from outside sources. Larger facilities may require employees to wash both their hands and feet for further decontamination.

Always remove all leaves, dying vegetation and dead vegetation from your growing environment. If standing water is not eliminated, the humidity levels of your growing environment will be influenced. Standing water can accelerate the growth rate powdery mildew.

Use HEPA filters and sterilize airflow

All incoming air should be filtered and sterilized. HEPA or high-efficiency particulate air filtration is recommended for all incoming air for both greenhouse and indoor growing environments.

The process of filtering air indoors will eliminate the potential of powdery mildew spores entering your growing environment. Filtration systems require meticulous maintenance. Once air has received filtration, sterilization is key. You can use UV or ultraviolet sterilization, an electronic system or another method.

Many different types of apparatuses and methods have been created specifically for sterilizing indoor air. Many greenhouse growers prefer to use bug screen filters specifically sized for air-intake sources to prevent pests from entering.

Decontaminate and maintain your environment with hydrogen peroxide

All equipment in your growing environment needs to be decontaminated and properly maintained using hydrogen peroxide. This includes good decontamination of all your exhaust and intake air-ventilation ducting. These actions need to be taken after the conclusion of every growth cycle.

Powdery Mildew Spores Cannabis
Ampelomyces are intracellular hyperparasites producing their asexual fruiting body pycnidia onside the powdery mildew condiophores. The fungus Ampelomyces quisqualis is a naturally occurring hyperparasite of both sexual and asexual stages of powdery mildew pathogens. It over parasitizes and produce pycnidia within powdery mildew hyphae, conidiophores and cleistothecia. The parasitized powdery mildew colonies are dull-white in appearance, flattened and off white to gray in color, with reduced conidial production. PHOTO CREDIT Dr Parthasarathy Seethapathy, Tamil Nadu Agricultural University,
Test and/or quarantine clones

Before purchasing your clones, take a specimen to a laboratory. You need to be certain no banned chemicals have been applied, such as myclobutanil, and that there is no powdery mildew on the clones.

This step is extremely important for every plant you purchase before introducing it into your growing environment. Your growing area needs to be quarantined, sterile and compartmentalized. This is the only way to make certain all your new specimens have received a clean bill of health prior to placing them in your growing facility. If you do not do so, you are risking a powdery mildew infection.

How to prevent Powdery Mildew on Cannabis Outdoors

  • Apply Trifecta Crop Control Super Concentrate as a preventative
  • Be aware of the season
  • Remove all dry leaves
  • Space plants far enough apart
  • Use biological phytosanitary treatments

You can prevent powdery mildew by applying Trifecta Crop Control weekly. Use 1oz for every gallon of water. To be effective, you need to consider both weather conditions and relative humidity levels. Powdery mildew in your outdoor plants is generally seasonal. Your greatest risk of powdery mildew is during the spring and autumn seasons.

Additional preventative measures can be taken. Remove all dry leaves around your plants. If your outdoor plants are in pots, turn your pots regularly to ensure every part of your plant is exposed to the sun. Make certain your plants are spaced far enough apart to ensure good air circulation. During the fall, biological phytosanitary treatments are recommended if your area is prone to mist and white fog.

Disease Cycle for Powdery Mildew

The complete lifecycle of PM includes both sexual and asexual reproduction. Asexual spores or conidia are produced by the asexual state. During the sexual state, ascocarps containing asci and ascospores are produced. This was previously referred to as chasmothecia, cleistothecia and perithecia. During the occurrence of both states, ascocarps are generally important to enable powdery mildew to survive without a living host.

Powdery Mildew Spores Cannabis
Powdery mildew and the naturally-occurring fungal hyperparasite, Ampelomyces quisqualis. The white mycelium and clear spores are of the powdery mildew fungus. The brown structures are the spore producing structures (pycnidia) of Ampelomyces quisqualis. PHOTO CREDIT Paul Bachi, University of Kentucky Research and Education Center,

During the conidial state, powdery mildew intensifies and spreads with incredibly large numbers produced throughout the growing season. The infection of the host requires between three and seven days provided the conditions are conducive for powdery mildew. The fast increase of asexual reproduction can result in populations of powdery mildew causing an epidemic.

Powdery mildew often sexually reproduces after flowering or toward the end of the growing season. Ascocarps have a natural resistance to drought and lower temperatures, enabling survival in the harshest of conditions. Sexual reproduction can cause genetic recombination resulting in the production of new genotypes with fungicide resistance. The new genotypes can have more resistance than the parental genotypes.

New epidemics can be initiated by ascocarps, generally occurring after the growing season or during the spring. After irrigation or a rainstorm, ascospores are often discharged. Ascospores in the soil on unprotected or susceptible host tissues often initiate an epidemic through the establishment of a primary infection.

How Long Does It Take for a Plant to Recover from Powdery Mildew?

One to two weeks are usually necessary for your plants to fully recover from powdery mildew.


Defeat Pests, Mold and Mildew… NATURALLY!

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