• Post last modified:July 6, 2022
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Whoever said that cultivating weed would be a walk in the park obviously has never had to deal with powdery mildew on cannabis plants. These plants come with a lot of responsibility, and growers are constantly worrying about the health of their plants. One thing that everyone looks forward to is the bountiful yield at the end of each growing season. Once you harvest those healthy-looking, THC-rich, flavorful, and aromatic buds, you realize that the struggle was worth it.

If you are an experienced grower will have probably heard or experienced the menace known as powdery mildew. This flour-like fungus is responsible for destroying many cannabis plants and is best known for its unique presentation as ‘white spots on cannabis leaves.’ It is one of the most prevalent pathogens when it comes to cannabis cultivation. If you are here, you must be looking for the cause, solution, or ways to prevent this fungus from getting to your prized plants. 

Well, you are in the right place. This article takes a deeper look into the causes of powdery mildew and how to deal with the fungus. You will also get access to tips to help you keep powdery mildew at bay.

What is powdery mildew?

Powdery mildew is a white, barrel-shaped pathogen caused by fungi called Ascomycetes, belonging to the Erysiphales order. The fungi affect plants like roses, cannabis, grapes, sunflowers, and others. It also goes by the monikers oidium, white mold, or simply PM. Powdery mildew affects both indoor and outdoor plants. 

PM survives by latching onto a live host. The parasite thrives in a humid, overcrowded, dark, and warm environment with still air. It affects newly forming leaves, although it can also attack older and more mature plants. The PM is prevalent during the vegetative phase but can also be a serious issue during the last few weeks of flowering and the drying and curing process.

It is relatively easy to identify, treat, and prevent PM. The key to fast and effective treatment is early detection. As a cannabis grower, regular inspection of your crop is essential. It is through these inspections that you can spot an issue early enough to save your yields.

What does powdery mildew look like?

Powdery mildew on cannabis is easy to identify, thanks to its conspicuous white appearance. Leaves with PM will have white or gray spots and will appear fuzzy. As the infection progresses, the leaves will become distorted and discolored. PM will first appear on leaves, and it tends to favor the upper part, although it can present underneath the leaves, hence the need for a thorough inspection. 

Besides the leaves, powdery mildew also affects the stems, branches, and buds of cannabis plants. In severe cases, plants with PM that have been neglected will eventually produce a repulsive, rotting odor.

What causes powdery mildew, and how does it occur?

During spring, daytime temperatures pass the 60°F mark, the ideal temperature for reproduction. The fungi will release spores during this time. Being an obligate parasite, the spores will remain in this condition until they can find a suitable host (your cannabis plants) to exploit and start to reproduce. 

Causes of powdery mildew

Multiple factors can cause powdery mildew, including;

  1. PM thrives in high humidity (70% and higher) and temperatures (68°F to 86°F)
  2. A poorly ventilated grow room with still air
  3. Contaminated and unfiltered air
  4. Clones that are infected with powdery mildew
  5. Unpruned, dense, and tightly packed plants
  6. Dark spots within the grow room due to inadequate light penetration to all plant parts.

How does powdery mildew reproduce?

Powdery mildew reproduces in either of two ways: sexually and asexually. 

Asexual reproduction produces conidia or asexual spores, which complete their growth cycle within three to seven days. If undetected, the PM reproduces rapidly to cause an infestation that becomes irreversible. Ultimately, you will lose a portion or all of your cannabis plants.

Sexual reproduction occurs when the PM genetic material is combined to produce ascocarps – offsprings with varying genetic characteristics. The diversity allows the fungus to develop resistance to abiotic factors such as high/low temperatures. Sexually produced PM produces genotypes that can quickly grow resistance to fungicides, making it a menace to deal with. Sexual production occurs as the growing season ends – late flowering stage and during drying and curing. 

How is powdery mildew spread?

Once the Powdery Mildew strikes, spreading it is very easy. As mentioned earlier, the fungi reproduce at an accelerated rate; thus, the multiple microscopic spores can spread and affect the entire plantation within a few days. Once the spores latch on the leaves, they disrupt normal and essential processes, such as the absorption of nutrients, photosynthesis, respiration, and transpiration. 

This ultimately leads to the death of leaves and plants, causing a massive loss to the cannabis cultivator. To avoid such instances, you need to be aware of the various ways that this deadly pathogen can spread. Below are some ways PM pores can be transferred from one cannabis plant to the next.

Through the air 

The PM microscopic spores can travel via the air from plant to plant, causing a pandemic within a short period. This situation will escalate much faster in a grow room with poor ventilation and unfiltered air. The pathogen gets sufficient time to find a host plant and complete the growth cycle.

Tools

When handling cannabis plants with powdery mildew, the fungi may attach to tools, such as scissors. The fungi transfer to other plants as you move to another grow section using the same scissors. It is advised to sterilize these tools regularly to avoid such instances. 

Grow room personnel

The Powdery Mildew can attach to the gloves or clothes of anyone handling the infected marijuana plants. The clothes and gloves provide the fungi with a free mode of transportation from plant to plant.

Stages of growth when cannabis plants are susceptible to powdery mildew

For outdoor-grown marijuana plants, cultivators need to be on the lookout during spring and autumn. This is when the plants are experiencing a growth spurt in the vegetative stage, heading to early bloom. PM may affect indoor-grown cannabis from the seedling ( infected clones) to the flowering stage.

The pathogen will thrive as long as optimum conditions are provided, explaining why some growers report powdery mildew during late flower stages. Additionally, it is not unusual to experience early-stage powdery mildew on dried buds.

How to cure powdery mildew on leaves

Although powdery mildew is destructive, early detection is vital in dealing with the situation. Do not hesitate to act once you notice the white coloration on the leaves, do not hesitate to act. Start by cutting off the infected leaves and placing them in a plastic bag. 

Be Careful not to release the spores in the air, leading to contamination. Tie the plastic bag containing the infected parts and dispose of it. Once done, decontaminate the equipment and change clothes in case the spores stick to them. 

As a result, you may need to take extra measures to ensure the health of your marijuana plants. Below are several natural remedies on how to get rid of powdery mildew on cannabis leaves;

Milk

Besides being an effective powdery mildew treatment option, milk contains vitamin B, beneficial proteins, calcium, and natural sugars that can help improve the health of your cannabis. Create a solution with 40% milk and 60% water and spray the eaves every 10 to 14 days. The solution is as effective as synthetic fungicides and sulfur and should start working within 24 hours.

Neem oil

Neem oil offers an excellent alternative to chemical fungicides, especially if the powdery mildew is in its early stages. Take 2.5 teaspoons of neem oil and mix it with a gallon of water. Apply once every 7 to 14 days and wait for your leaves to regain their health.

neem oil to treat powdery mildew on cannabis

Potassium bicarbonate 

Mix 1 tablespoon of potassium bicarbonate, half a teaspoon of non-detergent liquid soap, and a gallon of clean water. The solution contains a PH that creates a non-conducive environment for powdery mildew. 

Apple cider vinegar

Apple cider vinegar contains acetic acid, a compound that fights powdery mildew fast. Mix 2 to 3 tablespoons of organic apple cider vinegar with a gallon of water. Spray the cannabis leaves every day for the first week, then alternate to spraying the plants every other day.

cider vinegar to treat powdery mildew on cannabis

NOTE: DO NOT spray the plants while grow lights are on. Doing so will burn and destroy the leaves, creating more damage than the powdery mildew. 

How to care for cannabis buds with powdery mildew

Dealing with powdery mildew in the cannabis flowering, harvesting, and drying stages can be tricky. You cannot use synthetic fungicides as they interfere with the overall quality of the bud’s flavor and aroma. Additionally, you cannot sell buds with powdery mildew as it can be dangerous to consumers’ health. 

Most cannabis consumers use the plant medically for various ailments. Exposure to toxins from powdery mildew infestation with their already compromised immune system is wrong and may lead to more irreversible damage. Spraying infested buds with the highlighted solutions may lead to mold and bud rot due to dampness. So, what can you do? 

The best method to deal with powdery mildew in the flowering is by applying any of the solutions mentioned above (milk, neem oil, potassium bicarbonate, or apple cider vinegar) in a paper towel or cloth and gently but thoroughly wiping every inch of the bud. Wipe the top and bottom parts of the leaves. This ensures the bud’s flavor, potency, and aroma are not compromised and that the flower is protected from bud rot and mold infestation. 

How to prevent powdery mildew

Powdery mildew is a fast-spreading pathogen that can destroy marijuana plants in a matter of weeks. Luckily, there are preventative measures you can take to ensure your crops are protected from the fungi in the future. The first step is to find yourself a strain that is resistant to powdery mildew, primarily if you have dealt with the pathogen during previous growing seasons. Strains like Northern Lights, Super Silver Haze, and Afgoo are excellent options. This is how to prevent powdery mildew indoors and outdoors.

Indoors

Put an air filtration and purification system in place

Install a high-efficiency particulate air filtration, aka HEPA, in the grow room to filter and purify the air by removing any molecule as small as 3 microns or more. A PM spore measures 25 to 75 microns, which means it will be effectively evacuated from the environment. 

Maintain optimum humidity and temperature levels

PM thrives in a humid, warm environment. Ensure the humidity and temperature are controlled to avoid providing the perfect reproducing environment to the pathogen. Use a humidifier to maintain optimum grow conditions for your cannabis plants. 

Combine this with sufficient airflow, and powdery mildew will never be an issue. Additionally, avoid overhead watering and only water the plants from the roots to reduce humidity. Lastly, get rid of standing water in the grow room.

Clean and decontaminate the grow room.

If you are operating a large grow room, create a changing room where you or the grow room personnel can change the clothes and put on sterilized attires before entering the grow room. This ensures the plants are not exposed to pathogens from outside the grow room. Also, conduct regular inspection, removing unnecessary things like dead leaves.

After each growth cycle, use hydrogen peroxide to sterilize tools and equipment, such as scissors, pruners, environment-measuring and controlling equipment, drying and curing jars, etc. This will include the air filtration system (intake and exhaust air system). 

Inspect and quarantine clones

Thoroughly inspect clones before planting to ensure they do not contain harmful chemicals or pathogens, such as powdery mildew. Do not mix other plants with the clones before placing them under quarantine, as you will be risking the health of your marijuana plants and, ultimately, your yields. 

Space your plants and prune them regularly

Congestion of plants, branches, and leaves blocks light penetration and unrestricted airflow, creating the perfect environment for powdery mildew to thrive. Space out the plants and exfoliate regularly. 

Grow mold resistant strains

Growing with the right type of strain for your environment can eliminate the vast majority of plant problems that can occur. For example, growing a dense indica dominant strain such as Big Bud when you live in a highly humid climate such as southern Mexico will most likely increase your risk of bud rot or powdery mildew occurring. Whereas if you were to grow a sativa dominant strain such as Amnesia Haze or Thai Stick that is used to growing in humid conditions you are much less likely to encounter these problems.

That is not to say that anybody growing in hot or humid conditions needs to grow sativa’s, it simply means that if you decide upon growing an indica dominant strain then you need to do your very best to emulate the environment that suits it by regulating your grow room climate.

Outdoors 

Space the plants – Create space between plants for free flow of air

Remove dry leaves – this ensures that the remaining leaves get sufficient light for optimum growth. Regular pruning is essential in facilitating optimum light penetration. 

Watch out for spring and autumn – these are the seasons when cannabis plants are highly susceptible to powdery mildew. Be vigilant and apply preventive measures to guarantee the health of your plants. 

Take away

Powdery mildew is a parasitic fungus that affects cannabis plants during different growth stages. It is one of the deadliest pathogens that can destroy your crop, leading to massive losses. However, when detected early, the situation is reversible, and you can salvage your marijuana plants for a bountiful harvest. It is recommended that you utilize preventative measures to ensure powdery mildew does not find any opportunity to compromise your plants.

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