Powdery mildew and cannabis

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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.

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.

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.

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).

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:

Fruity Jack / Jack El Frutero
Amnesika 2.0
Early Maroc
Easy Haze / K-13 Haze
Sweet Love
Spicy CBD / Superjuani

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.

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.