Cannabis Pests - Powdery Mildew

Mold is a dreaded enemy of any cannabis cultivator. Fungus spreads very quickly if left unattended and can destroy whole crops in a flash. The best way to avoid mold is to keep the proper conditions, and preventing powdery mildew is no exception. This guide discusses the causes, symptoms and treatment of powdery mildew, as well as some tips to prevent it.

What Is Powdery Mildew?

Powdery mildew is a fungal mold that appears as a white powdery substance on many plants, including cannabis. Once powdery mildew has established itself, black spots can develop amongst the white powder as the mildew starts heavily reproducing by spreading spores. While the fungus multiplies it can spread over entire areas, preventing the plant from photosynthesising and essentially suffocating it.

The Causes Of Powdery Mildew

High temperatures combined with high humidity and bad air circulation are usually the leading causes of powdery mildew. Mold spores are always present in the air, and when the conditions are favourable they will settle and start germinating. When air is not circulating properly, pockets of hot, humid air can build up around the plant, creating the ideal environment for mold to get cosy. This can make mold quite difficult to combat, as it can strike at any moment when there is an imbalance in the environment.

Nutrient levels also have an impact on powdery mildew formation. For instance, if a cannabis plant is experiencing a deficiency or other nutrient related problem, functions slow down and the plant's defence system is weakened. When the plant is not healthy, it provides an opportunity for mold to start feasting.

Causes of powdery mildew:

• Poor airflow - Pockets of stagnant, humid air building up around the plant creates an inviting environment for mold. Usually from a lack of fans and proper ventilation.
• High humidity - Levels over 60% may be dangerous for cannabis plants, especially in flowering and when combined with warm temperatures.
• Overcrowded grow space - Your plants could be grouped too closely together and/or there is too much shaded leaf growth. This increases the chance of the mold spreading more quickly.
• Overwatering - If the substrate stays moist for extended periods of time there is an increased risk of powdery mildew. Outdoors, damp weather, rain and dew can also be the cause.

Symptoms of Powdery Mildew

Powdery mildew is fairly obvious when it does show and is recognised as a white, floury powder that primarily attacks the leaves. Unfortunately, because mold spores are so tiny, it is very hard to detect an infestation early on. Spores can be dormant for months before they have a chance to multiply.

Powdery mildew may appear at any stage in a cannabis plant's life, however it is obviously more problematic during flowering and can be tougher to treat without damaging the buds. Let's take a look at how to identify powdery mildew before it reaches that stage.

Symptoms of Powdery Mildew:

• Fine, white, powdery substance on leaves
• Shrivelled/darkening leaves
• Slow growth
• Black spots appear when the fungus starts heavily multiplying
• Leaves curl upwards and rot
• Buds may be attacked, eventually killing a plant.

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You may also detect powdery mildew around younger leaves where they are grouped at the tip of the new growth.

Treating Powdery Mildew

Thankfully, treating powdery mildew is relatively easy when the infestation isn't severe, and there is plenty we can do to get rid of it before it takes over. Be careful when handling infected plants, as the spores are very light and can be transported easily. If your plants are already well into flowering, there is little time to save them so consider your options.

• Correct the conditions - One of the first things to do when you notice powdery mildew is to check the cause. Applying treatments before you have corrected the environment may mean the disease comes back more easily. Adjust the pH, nutrients, temperature and humidity and switch of oscillating fans while you treat, if possible.
• Remove any infected foliage and discard it - This is the quickest way to remove the mold. The leaves are likely to die off eventually so the sooner they are removed, the sooner your plant can redirect its focus to maintain health at the newer growth. Powdery mildew feeds off dying plant material.
• Keep a clean environment - Make sure to keep your grow space as clean as possible and sterilise everything between each cycle. During the grow, routinely disinfect surfaces and tools using cold water and alcohol (isopropyl). Even when there isn't a mold problem, it's best to remove any yellowing/browning leaves so we don't give powdery mildew a reason to settle.

• Alkaline water - Powdery mildew likes acidic environments so rubbing it off with a cold sponge and water at pH 8-9 can help to kill off a large portion of the colony. This can be a good way to start treatment before applying any sprays. Alkaline water kills the spores.
• Natural fungicides - Diluted milk, baking soda or household vinegar (like apple cider vinegar) can be used as foliar spray to treat powdery mildew.
• Neem oil - A powerful antifungal and antibacterial product used to treat many pest problems. More effective as a preventative measure but can be applied as a foliar spray (with insecticidal soap and water) every week to prevent spores from reproducing. Like most sprays, avoid using during the last 3-4 weeks of flowering and try not to spray on buds.
• Hydrogen peroxide - You can prepare a solution using 3% strength hydrogen peroxide (H202) for an effective, non-toxic treatment for powdery mildew. Dilute at 1 part 3% H202 with 3 parts of water. Mix into a spray bottle and shake. Spray each morning before the lights come on until the mold dies off.

Tip: When making foliar sprays, it's best to make a fresh solution each time you treat the plants. As you spray, thoroughly coat all sides of the leaves until the moisture drips off.

Outdoors

Powdery mildew affects many plants outside and cannabis is no different. Hot, dry weather can be an issue when there is little wind involved. It can be hard to avoid so be sure to monitor and prepare for the weather. Check your plants daily for signs so you can find a solution quickly.

In winter, the lack of light and colder conditions (which usually come with high humidity) can be a challenge. Greenhouse cannabis plants can be particularly susceptible to powdery mildew due to the high humidity levels that tend to build up in the space. Try to add some fans or see if it's possible to install a dehumidifier.

Powdery mildew on greenflipcali89's cannabis plants.

Tips To Prevent Powdery Mildew

To finish off, here are a bunch of tips to experiment with when preventing mold problems such as powdery mildew. It may seem like an effort to be worrying about something that isn't visibly there, but preparing for the worst can ultimately save your harvest from failure.

• UV light has shown to kill mold spores
• Maintain relative humidity between 45-60%
• Avoid letting temperatures exceed 24-25°C, unless you're using supplementary CO2
• Consider installing a dehumidifier to remove moisture
• Space out your cannabis plants and regularly tidy up dead leaves
• Regularly check for pests
• Choose mold resistant strains that can handle training
• Add more silicon into feeding schedule to strengthen plants
• Apply preventative foliar spray (e.g. Neem oil) periodically during vegetation.
• Sterilize equipment with hydrogen peroxide.

Conclusion

Remember, the key to stopping molds like powdery mildew is to prevent them in every way you can. With the help of this article, we hope you manage to find a solution to this destructive pathogen.

If you found this guide useful, feel free to share your thoughts down in the comments section! The more growers know about preventing molds, the better. Happy growing!

External References

Differentiating Powdery Mildew from False Powdery Mildew. Journal of Industrial Hemp. - McPartland, John & Hillig, Karl. (2008).

Fungicide Resistance in Powdery Mildew Fungi. Microorganisms. - Vielba Fernández, Alejandra & Polonio, Alvaro & Ruiz-Jiménez, Laura & Vicente, Antonio & Pérez-García, Alejandro & Fernández-Ortuño, Dolores. (2020).

Cucurbit Powdery Mildew in the USA. - McGrath, Margaret. (2015).

Effect of weather parameters on development of ber powdery mildew and its control by fungicides. - Amrate, Pawan & Singh, Amarjit & Mohan, Chander. (2020).

Effects of water on the development of powdery mildews. - Sivapalan, A.. (1981). 

Genetics of Powdery Mildews. Annual Review of Phytopathology. - Moseman, J. (2003).

This article was updated November 2020.