Powdery mildew and cannabis

Powdery Mildew ia a fungal disease which mainly attacks plant leaves. There are many species that can be attacked by Powdery Mildew, such as roses, carnations, tomatos, strawberries, cucurbits, apple trees, plum trees and, of course, cannabis. It can be devastating for marijuana if not treated early, since it reaches the buds and ruins its flavour.

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Powdery Mildew

The most dangerous Powdery Mildew strains for cannabis plants are Sphaerotheca macularis and Leveillula taurica, which should not be confused with downy mildew, which also attacks the leafs of the plants but, in this case, mainly on the underside of the leaves - while powdery mildew normally infects the surface, although it depends on strains.

Let's see now the evolution of a powdery mildew infection and what can we do to treat it.

How to identify a powdery mildew infection in cannabis

Detecting this pest is easy, since we'll quickly notice the growth of white powder stains with cotton like texture on the surface of the largest leaves (formed by the mycelium). These stains are small at first, but if growth conditions are optimal and the infection is not treated they'll soon cover most of the leaves, even the small sugar leaves of the buds.

At the end of this post you'll find a picture gallery that will make the identification of powdery mildew much easier.

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Cannabis infected with powdery mildew (Picture: Vincent from Alchimia)

As often happens, the presence of this fungus depends on several factors, such as the environment (temperature and humidity), light intensity, nitrogen amount, cannabis variety, etc. Most powdery mildew strains like temperatures between 15 and 28ºC (59ºF-82ºF) and relatively high humidity levels, from 70%.

Spores land on the leaves and, if growth conditions are optimal, start germinating. A thin filament grows from the spores and pierces the plant tissues, absorbing nutrients from the plant cells. Low/high humidity cycles (for example, morning dew and low humidity during the day) are ideal conditions for the germination (high humidity) and growth (low humidity on the leaves) of powdery mildew spores. Thus, poorly ventilated areas - greenhouses, etc. - are perfect for its propagation.

We must also pay attention in indoor grow rooms, where this fungus can quickly propagate if growth conditions are met, also in greenhouses and outdoor crops where environmental conditions often enable its development. Notice that this plant pathogen can complete its life cycle in just 5 days.

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First symptoms of powdery mildew

The weakest plants are more sensitive to powdery mildew attacks, as well as those with too much foliar mass or placed in shady areas.

Powdery mildew spores mainly move thanks to aphids and wind, although humans can also act as vectors of this disease.

Prevention of powdery mildew in cannabis plants

First, we should always make sure that our cuttings or seedlings are healthy and free from pests. While this is forgotten most times, it is really important if we want a quality crop. Almost any pest will ruin our product if not treated early and properly.

If we are growing indoors, our growing space should be clean and free from dead plant matter, etc. Humidity levels between 40 and 65% and proper ventilation are very important. Do not wet the leaves unless it's strictly necessary. If we know that there's powdery mildew in our area, using spore-proof filters - HEPA - in our ventilation system greatly reduces the possibilities of an infection inside our grow, as well as removing the lower branches and using some type of fungicide regularly (never spray your plants after the first 2 weeks into bloom).

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First symptoms of powdery mildew

If we are growing outdoors, it is always important to know if the other plants around our cannabis plants are sensitive to powdery mildew. Normally, this fungi propagates during spring and autumn. If we see traces of powdery mildew in other plants, then we know that we'll probably have to use some preventive fungicide to protect our plants, otherwise they'll probably be infected sometime during the bloom period. Whenever possible, we should cover our plants in case of hard rains or regular morning dew and always leave enough space between them to improve ventilation. Rainy areas, with high humidity levels and mist are ideal for powdery mildew propagation.

Of course, using strains especially resistant to fungi is recommended. The most resistant varieties to powdery mildew from Philosopher Seeds catalogue are:

Treating powdery mildew in cannabis plants

Once the plant is infected, natural fungicides are not very useful, so we'll have to use systemic fungicides- much more effective - like penconazole (always follow the instructions and respect the safety deadline of the product). Whenever possible, we should adjust the climate conditions of our grow for not favouring any infection.

During the growth phase, we can use horsetail, propolis, potassium soap or sulphur as preventives, also to treat the first symptoms of an infection. As we mentioned, we can also use systemic fungicides, which remain in the plant tissues and protect it for several weeks. Many growers use this type of fungicides right before switching to bloom, so that plants will be protected during the first weeks of this crucial stage. In this way, they greatly reduce the chances of major infections.

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Prevention is crucial to avoid major issues

During the flowering period we should always try to avoid using any kinf of plant-protection product. Still, if our plants are infected, we strongly recommend to use these products exclusively during the first 2-3 weeks of bloom, otherwise the flavour of the buds can be ruined, even if we make resin extractions. Both sulphur and potassium soap can leave residues, so we should not use them during this stage.

As we already mentioned, always try to start your crops with strong, healthy plants, it is the best way to avoid future problems. Keeping your mother plants and cuttings/seedling healthy is the best way to ensure a quality harvest.

We hope this post helped you, if you have further doubts or comments please leave them here, we'll be pleased to reply them!

Have a nice smoke!

Comments in “Powdery mildew and cannabis” (4)


Kasia 2020-01-26
I have suffered adverse effects from trimming powdery mildew off of dry buds. I had a headache, dry cough, and tight lungs for about 3 days after 5+ hours of exposure. It happened once before as well after trimming powdery mildew buds so I am certain this is what has caused my reaction.


Isaac Henderson 2019-08-16
Just noticed a couple spots on my plant, I was careful to do a lot of defoliation of the affected leaves to help air flow and I have been doing routine sprayings of the plants, could this fungus carry into extracts if it is somehow present in the buds? Are the buds safe to smoke as long as there is no closely affected leaves or affected buds? At what point is the finished product no longer safe for consumption? Any feedback would be greatly appreciated!!

Philosopher seeds Staff

Dani 2019-09-12
Hi Isaac, Yes, the fungus spore may contaminate your extract. The best thing you can do is mix the extract with ethanol and filter it with a 0,2 micron syringe filter, which should leave your extract clean from spores. Remember to purge the extract then to remove any trace of ethanol! Best!


JIAN YANG 2019-05-22
I am interested to buy some seeds of your PW resistant strain. Thanks.

Philosopher seeds Staff

Dani 2019-05-24
Hello, There's no problem at all! You can check your delivery zone here. You can place your order through our website or by sending us an email to info@philosopherseeds.com Best!


Prof Y Cohen 2018-03-20
I am Professor of Plant Pathology in Israel I wish to buy/obtain a few seeds of your powdery mildew resistant cannabis strains. How can I do it??

Philosopher seeds Staff

Dani 2018-04-03
Hi Prof Y Cohen, I'm replying via email. Best!

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About this Cannabis Blog

This is the blog of the seed bank Philosopher Seeds. It is intended for the use of adults over the age of 18 years.

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