Mildew is one of the pathogens most commonly found in cannabis crops. It usually emerges when a series of conditions that favour its development occur at the same time. In this post we explain how to identify this fungus, under what circumstances it may appear, and, most importantly, how to eradicate it.
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Mildew in a nutshell
Mildew first appeared in Europe in 1878, when the French started to import American grapevine strains that were resistant to phylloxera, a plague that caused havoc in the French wine industry of the time.
Furthermore, the relevance of these new American species facilitated the arrival of mildew to the rest of Europe. This pathogen can be detrimental to many plant species, especially fruit and vegetables, and therefore became a real nightmare for European and worldwide farmers alike.
How to detect mildew
Mildew is a common pest in cannabis crops across the globe. It discreetly fixes to vegetable tissue, safely under cover, and has the ability to thrive without drawing much attention.
It settles in leaves, stems, and even in the plant’s flowers or fruits. The hyphae (a network of filaments that make up the structure of the fungus) can firmly fix to these organs, where the fungi can happily grow and propagate.
Why do we say that mildew is a discreet strategist? Because growers often spot it when it’s far too late, which is what usually occurs with most endoparasites: these only show symptoms on the aerial parts of the plants at very advanced stages of growth.
How does mildew manifest itself?
Most common symptoms include whitish or yellowish spots on the leaves, similar to translucent oil stains. If there’s too much moisture, a white powder can also appear on the underside of the leaves. However, it’s important not to mistake this for powdery mildew, which can show similar symptoms.
As the infection spreads, these stains are confined by the veins of the leaves and become darker until they turn brown. When moisture levels are high, they get coated in a fine layer of white fluffy dust.
Infection can also be found on the stem, with the appearance of dark stains that can eventually damage the whole area as infection advances. If no action is taken right away to curb the spread of the disease, the affected area will slowly wither away.
During the following phase of infection, the affected areas die and the leaves fall off prematurely, which translates into a decrease in cannabis yield and quality.
Favourable conditions for the appearance and propagation of mildew
Mildew is a deceptive fungus for several reasons. In addition to developing discreetly in areas not visible at first sight, it can remain dormant for some time before becoming active again.
When conditions are favourable for its growth, microorganisms can remain dormant during autumn and winter, when they’re found in spore or mycelium form (fungus structure at a late stage).
In spring, when temperature and humidity levels rise, these microorganisms usually come to life again. Mildew requires a temperature between 10 and 25⁰C to develop optimally.
Regardless whether you're growing indoors or outdoors, the higher the humidity level and temperature, the quicker and more severe the propagation and development of the fungus will be.
Preventing its appearance
There are some guidelines that you can follow to prevent the appearance of this pathogen on your cannabis crop:
- Keep your grow space clean.
- Keep your crop free of plant litter.
- Minimise air humidity (good management of indoor ventilation and temperature) as well as during watering.
- Minimise heat (in indoor spaces) and light exposure (lighting).
If you don’t control these risk factors, there are more chances that this fungus could appear in your grow. It’s much easier to follow these guidelines in indoor spaces than outdoors, where the actions of a grower are more limited. Outdoors, if your plants become affected by mildew, limit irrigation frequency and quantity.
Preventive treatment against mildew
If the grow conditions can lead to the appearance of mildew, you can spray your cannabis plants with a garlic or chamomile infusion.
You can also use fungicides like dithiocarbamate, copper oxychloride, or organic compounds such as Bordeaux mixture (copper sulphate), or copper salts. These products are very effective fungicides. You can also purchase these products in our online grow shop, where you’ll find a great range of brands like Prot-Eco or Trabe.
Healing treatment for mildew
If mildew manages to reach an advanced stage of development, the only way to eradicate it and save your cannabis crop is by applying effective chemical treatments.
You can choose between two types of chemical treatments and two very different courses of action.
The first one is based on fungicides that act by contact (they’re sprayed on the plant tissue): these include chlorothanolil, cymoxanil, and imazalil.
The second option comprises systemic fungicides (which are added to the nutrient solution): triadimenol, metalaxyl, and myclobutanil. Before use, it’s important to learn the specific directions for each product. This includes dosage levels, effect duration, and also required safety measures in order to minimise health risks.