Cannabis cultivation comes with its fair share of challenges, powdery mildew being at the top of the list. The appearance of white spots on cannabis leaves is enough to send any cultivator into panic mode. This is because the fungus can quickly destroy the entire growth cycle of cannabis plants if not dealt with early enough. Powdery Mildew, aka Oidium or white mold, is a common fungus belonging to the Ascomycetes fungi, a classification of the Erisiphales fungi order. 

Powdery mildew on cannabis is a widespread problem affecting greenhouse, indoor, and outdoor-grown plants. The pathogen can also be found in tomatoes, cucumbers, melons, peppers, and many other plants. If you are dealing with powdery mildew or researching the pathogen in case it ever strikes, you are in the right place. This piece features everything you need to know about powdery mildew, including tips on how to prevent its occurrence in the first place.

What is powdery mildew, and how does it occur?

Powdery Mildew is a fungal pathogen that initially presents as white spots on cannabis leaves. It tends to favor the upper side of leaves but can also attack the bottom. When not handled early enough, PM spreads to other plant parts, such as the stems, petioles, and buds. The pathogen can strike during any growth stage or during the drying and curing process. 

When Powdery mildew is introduced into a grow room (more on how this happens later on), the spores can stay dormant for a long time, waiting for the perfect conditions to present. When this happens, it quickly latches onto its host and starts to reproduce. The fungi thrive in temperatures ranging from 68° to 86° and humidity of 70% or higher. It multiplies exponentially when attacking plants in an overcrowded, dark, poorly ventilated, and warm environment so it is vital to control your grow room climate as a preventative measure. 

Powdery mildew on cannabis usually targets the newly formed leaves first and later spreads to the older ones located at the bottom. It attacks by creating a white layer over the cannabis leaves, thus preventing them from getting enough sunlight for photosynthesis. The PM coating obstructs the respiration and transpiration processes, leading to a weak plant and stunted growth. Eventually, the leaves dry out and fall off the plant. When the plant loses leaves, its ability to produce enough energy for its survival is compromised. Consequently, the plant dies, leaving the cultivator with a considerable loss.

How to spot powdery mildew on cannabis plants

Under a microscope, it appears as barrel-shaped, colorless fungi that barely grows larger than 0.02mm. The pathogen becomes visible to the naked eye in large quantities as the spores form chains. You can spot small, white, or gray patches that take on a circular shape at the initial stage of the attack. As the spores continue to reproduce, Oidium appears as a layer of flour on leaves, almost as if someone poured baby powder on them. Plants with powdery mildew will have wilting leaves with bright green or yellow spots. Young leaves will appear discolored and distorted and will experience limited growth.

powdery mildew on cannabis
Powdery Mildew late flower

How significant a damage can white powdery mildew cause?

Early detection can be instrumental in salvaging your crop if you happen to find powdery mildew on your cannabis plants during the seedling or vegetative stage. If not detected early, it can lead to the death of seedlings due to their vulnerability. Plants in the vegetative stage may recover after thorough recovery techniques, although this can cause lasting damage and significantly increase the time your grow will take. The advantage cannabis plants have have in dealing with fungal infections like powdery mildew is that they can easily grow new leaves to replace the compromised ones, thus providing enough energy for the plants to still produce an impressive yield if caught early. 

When powdery mildew strikes during the flowering stage, it can significantly affect the overall quality of the bud and yield. The plant is no longer growing but focusing on bud creation. Powdery mildew may dramatically affect the final yield, causing severe losses to the cultivator. 

Powdery mildew on dried buds is something that every cultivator dreads. There are limited options on how you can deal with such a situation (discussed later on). However, it is possible to salvage the situation, although you may lose some of the harvests due to compromised, unsmokable bud. 

How does powdery mildew reproduce, and what conditions are necessary to thrive?

Powdery mildew’s life cycle combines both asexual and sexual reproduction. The former occurs during the cannabis plant’s growth period from seedling up to the flowering period or as the growing session ends. The pathogen produces conidia or asexual spores at this stage, which are obligate parasites that require a live host for survival. The asexual spores spread rapidly and take between three to seven days to infect the cannabis plants. It poses a threat in this state as it can easily cause a pandemic that is hard to come back from.

Sexual reproduction of powdery mildew occurs during the last stages of cannabis growth. This type of reproduction produces spores (ascocarps) that can survive without a host; that is why you can spot powdery mildew late flower or even early-stage powdery mildew on dried buds. Sexual spores are highly resistant to humidity and temperatures, enabling them to survive harsh conditions. The Ascocarps vary in genetic composition as they result from a combination of two parent spores. They can develop resistance to fungicides, making them nearly impossible to combat.

As mentioned earlier, the fungi thrive in temperatures of 68°F to 86°F, similar to the ideal temperature for cannabis cultivation. Another contributing factor is high humidity of 70% and above. When a grow room experiences temperature fluctuations, the relative humidity changes. A change from a low to a high temperature (even within the normal 68°F to 86°F) will increase relative humidity. When the humidity is high, it presents the perfect environment for powdery mildew spore reproduction. This explains why the pathogen is prevalent in cannabis cultivation grow environments. Other factors within the grow room that can support the spread of powdery mildew include;

  • Poor ventilation leading to areas with stagnant air
  • Contaminated and unfiltered air
  • Dark spots and tightly packed plants

Factors facilitating the spread of powdery mildew

Powdery mildew is introduced into the grow room in various ways. The introduction of infected clones is one of the leading causes. Another cause is powdery mildew from other plants. When gardening, one of the plants might have powdery mildew in its early stages. One can spread the pathogen to cannabis plants without knowing, especially if they do not decontaminate or change clothes before entering the grow room. Lastly, it is likely to reoccur if you have dealt with it in the past and not thoroughly decontaminated and sterilized the grow room. The pathogen’s spore remains dormant until the ideal conditions for reproduction are present. Once powdery mildew strikes, the factors highlighted below can hasten the spread at an alarming rate. 

Tightly spaced plants

Plants that grow close to each other make it easy for the fungi to spread. This can be further exacerbated if the plants are dense and bushy. Cannabis plants should ideally be spaced such that they can fully stretch laterally without touching other plants. 

Cross-contamination

Powdery mildew can be spread via contaminated tools. The spores attach to equipment, such as pruners, and latch on to healthy plants when used afterward. This is a dangerous practice that can quickly destroy crops grown in different areas. 

Another way cross-contamination happens is when the grow room personnel carries spores on their clothes from a contaminated area to a healthy section of cannabis plants. 

Poor ventilation

A grow room/space should be well ventilated as still air gives the pathogen sufficient time to latch on the leaves, complete its growth cycle,  reproduce, and become a pandemic. 

How to get rid of powdery mildew on cannabis

Cannabis leaves

Powdery mildew has struck. What can you do? First, you need to determine the severity of the fungal infestation. If it’s mild, you can utilize one of the following natural ways of dealing with powdery mildew. NOTE: Spray the leaves with these solutions when the lights are off to avoid burning them.

how to get rid of powdery mildew on cannabis
Spraying powdery mildew on cannabis with one of these solutions is the most effective treatment

Potassium bicarbonate

Potassium bicarbonate has a high PH that discourages the spread or growth of powdery mildew. This solution will kill the fungi within 2-4 weeks, leaving them as healthy as they should be. To make a solution, take a tablespoon of potassium bicarbonate and mix it with a teaspoon of liquid soap (non-detergent) and one gallon of purified water. Apply daily until the PM disappears. 

Apple cider vinegar

Acetic acid is abundantly present in apple cider vinegar. This compound is known for its fast-acting properties against powdery mildew. Take 2 or 3 tablespoons of natural apple cider vinegar and mix it with one gallon of water. Spray the affected plant with this solution for seven days, then do it every other day until the leaves regain their original color.  

Neem oil

Neem oil is an excellent option for anyone dealing with the initial stages of PM infestation. The solution is mild but tough enough to correct the issue within a week or two. Use two and a half teaspoons of neem oil and a gallon of water to make the solution. Use the solution regularly until the PM issue is corrected.

Milk

Readily available, cheap, and easy to make. Milk also contains numerous beneficial compounds that improve the health of cannabis plants. Using a 60/40 water-milk ratio, create your own PM cure. You will notice your plants regaining their health and vitality within 24 hours.

Advanced infestation  

If the pathogen has caused significant damage to the plants, cut out the affected leaves and put them inside a seal bag. Take caution so as not to spread the spores to other plants. Once you are left with healthy or mildly affected leaves, try out any of the highlighted solutions above to get rid of the PM that might be present.

There are times when one detects powdery mildew after it has done irreversible damage to the plants. In such a case, you need to count your losses and get rid of all the plants. Properly dispose of them and, if possible, burn them. Do not use the plants as compost since the spores might destroy your next crop. 

Cannabis buds

Powdery mildew-infested buds are very fragile and hard to deal with. As a cultivator, your ultimate goal is to produce top-shelf buds with a high degree of aromatic scents and full of flavors. Using fungicides interferes with these qualities and lowers the overall quality of the yield. Unfortunately, you can’t also sell or smoke the buds as they pose a serious health risk. 

The only way to kill powdery mildew on cannabis is by using one of the solutions highlighted above. Take a neem oil, potassium bicarbonate, apple cider vinegar, or milk solution and spray it on a piece of cloth. Ensure it isn’t drenched in the solution to avoid dampness which may result in bud rot. Wipe the fan and sugar leaves and any part of the buds with signs of the fungi. Afterward, ensure you subject the buds to the ideal conditions for proper drying to keep moisture away. Do this regularly until the PM can no longer be spotted. 

Preventing powdery mildew

Before you begin to cultivate cannabis, you need to select a strain that is resistant to common biotic and abiotic factors. Powdery mildew resistance is a quality that is desired by many cultivators, especially those that have faced the issue before. Buy seeds like Northern Lights, Gorilla Glue, and Afgoo from a reputable company that guarantees a 99% germination rate.    

The first line of defence against powdery mildew is prevention. As you cultivate cannabis, you need to put measures into place to ensure the entire process is seamless and results in healthy plants and a bountiful harvest. Here is how you can prevent PM

Indoors 

  • Only buy clean, tested, and powdery mildew-free clones. Get them from reliable,  trusted sources that other cultivators can verify. After purchasing, ensure you decontaminate the clones before installation. 
  • Ensure the grow room is positively pressurized. This guarantees that the air is pushed outside rather than indoors when the door is opened. When the opposite happens in a negatively pressurized room, the air coming in may contain pathogens that may compromise the health of the plants. 
  • Maintain steady humidity levels. The optimum humidity for cannabis growth is between 40% and 70%. Avoid stagnant water or overhead watering as they increase the air moisture levels. Utilize dehumidifiers to keep the levels in check. 
  • Increased airflow may dry the leaves and break them, along with some small branches. Inadequate airflow in the grow room creates the perfect environment for PM proliferation. An ideal grow room should have a good setup airflow system that supports a 13-mph breeze. 
  • Utilize a HEPA (High-Efficiency Particulate Air filtration) system. This will keep the powdery mildew spores out of the room. The filtered air can then be sterilized electronically or via UV sterilization. Ensure you maintain all your grow room systems for optimum performance. 
  • Conduct regular maintenance of tools and equipment used in the gow room. Additionally, decontaminate them using hydrogen peroxide to prevent pathogen transfers from plant to plant. 
  • Focus on keeping the grow room clean by removing dead leaves or decaying plant matter. After every growth cycle, clean the entire grow room and decontaminate surfaces and tools thoroughly. 
  • All personnel entering the grow room should decontaminate and change into work clothes before entering the grow room. This prevents any pests or pathogens from invading the prized plants. 
  • Keep your plants spaced out. Closely packed plants encourage the spread and reproduction of powdery mildew. Incorporate trimming and pruning practices into your cultivation schedule. 

Outdoors

Growing cannabis in an open setup can be challenging due to the range by which one can control the environment. To avoid powdery mildew, ensure that all your plants are spaced out. Outdoor-grown plants tend to be bigger compared to indoor-grown ones. You need to ensure they have sufficient space to stretch out laterally and vertically. 

Occasionally prune the plants to avoid a bushy structure. This ensures that all plant parts are exposed to optimum sunlight for impressive yields. Lastly, take extra precautions during spring and autumn. During these seasons, cannabis plants are highly susceptible to powdery mildew.

Take away

Powdery mildew is one of the many cannabis leaf problems cultivators face. The pathogen can easily destroy an entire crop if detected late. Conducting a thorough inspection of plants is warranted as it allows growers to identify such issues at their initial stage of an attack and deal with them instantaneously.

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