Learn to identify and combat the most common fungi trio in cannabis: powdery mildew, downy mildew and botrytis

Among the most common fungi that can affect cannabis plants are powdery mildew, downy mildew, and botrytis. If these fungi aren’t controlled in time, they can cause devastating damage to your grow. In this article, we show you how to identify each one of them, and we also reveal effective and sustainable strategies to treat them and to prevent their appearance.

In cannabis growing, fungi are one of the worst pests that growers can come across. It is highly likely that you’ll have to deal with some of them sooner or later.

Therefore, spotting them at an early stage, as well as knowing exactly what kind of fungus you’re dealing with, will be key to avoiding irreversible damage. Here we explain the different characteristics of each of these pathogens.

What’s powdery mildew and how to identify it in cannabis plants?

Powdery mildew is a fungus that’s easy to detect and can appear in both indoor and outdoor grows. It is vital to watch for its first signs, or it could otherwise render your entire crop unusable: using cannabis infected with powdery mildew can cause serious health problems. This pathogen goes through different phases as it colonises your marijuana plants:

• At the initial stage, powdery mildew manifests as small spots of a whitish powder that appear on top of the leaves, usually on those located in the lower area of the plant.
• As powdery mildew advances, these ‘spots’ are more visible as they become larger and their white colour becomes more intense.
• When this fungus is at a very advanced stage of development, the ‘white powder’ spreads to the stems and buds of marijuana plants. Having reached this point, the plants are no longer fit for consumption.

What is downy mildew and how to recognise it?

Downy mildew and powdery mildew can be easily confused with each other because they produce similar symptoms. However, there are certain differences between them that you must learn to detect so you know which enemy you’re facing:

• At first, downy mildew may appear in the form of whitish or yellowish (and somewhat translucent) spots on the leaves.
• Depending on the ambient humidity levels, a white powder (like that of powdery mildew) may appear. However, downy mildew usually appears on the underside of the leaves rather than on the surface.
• Another distinctive symptom of downy mildew is that it creates discoloured and even necrosed areas on the leaves.
• As the infection progresses, the discoloured spots change from yellow to brown. If allowed to evolve, the leaves will eventually dry and fall.
• In contrast with powdery mildew, downy mildew can also affect the stems of cannabis plants in the form of brown spots.

What is botrytis and how does it manifest in cannabis plants?

Botrytis is easier to distinguish than the two previous fungi. Unlike powdery mildew and downy mildew, botrytis can affect any part of the plant: roots, stems, leaves, or buds. This fungus shows very characteristic signs that are easy to detect:

• The affected parts of the plant become necrosed, showing a dry, lifeless appearance, and turning brown or grey.
• If this fungus has spread widely, you’ll see how it develops profusely, forming an ash grey mycelium. At that point, the affected part will have died and will fall apart when squeezed (don’t touch the infected parts directly with your hands; always wear gloves and dispose of them after use).
• It is common for botrytis to attack the buds in the final stages of flowering, especially in grows with high humidity levels or with strains that produce dense and compact buds.
• When the drying process isn’t performed correctly, botrytis can also appear showing the same symptoms. Regardless of which stage this fungus appears, remember that the affected flowers are no longer fit for consumption, as they could lead to serious health problems.

How to prevent powdery mildew, downy mildew, and botrytis in cannabis grows

There is a golden rule in cannabis growing: it is better to prevent than to cure. Here’s a detailed account of the steps you should take to stop these intruders from attacking your cannabis plants:

• In indoor grows, keep humidity levels between 40 and 60%. Appropriate and constant extraction of air from your grow room or grow tent is also essential.
• Set up a good ventilation system inside your indoor grow to avoid moisture pockets.
• Prune the lower parts of the plants which aren’t productive due to low light exposure.
• Exercise caution in your outdoor grow with the arrival of the rainy months.
• Keep your grow clean. Remove dry leaves and plant matter from the ground.
• Leave space between the plants to ensure proper air circulation.

How to eliminate powdery mildew, downy mildew, and botrytis from your cannabis plants

If any of these fungi has unfortunately appeared in your grow, acting promptly can make a huge difference.

How to treat powdery mildew in cannabis grows

• Apply horsetail spray: its high silica and equisetonine content can be effective against this fungus.
• Use biological fungicides: you’ll find effective products to eliminate powdery mildew in any grow shop. We advise you to avoid using non-biological fungicides; but should you decide to use them, make sure you do so while the plants are in their vegetative stage.
• If your plants are already in the flowering phase, you can spray them with a solution of 90% water and 10% milk. During this stage, you can also eliminate powdery mildew with a mix of water and a little hydrogen peroxide.

How to eliminate downy mildew from your cannabis plants

• Eliminate the affected areas and try your best in terms of hygiene so you don’t spread the fungus to the healthy plants.
• Apply organic fungicides based on horsetail or bacillus. There are also chemical fungicides available to eradicate downy mildew (both by contact and systemic), but we don’t recommend these: remember that everything you apply to your plants will end up in your body when you use cannabis.
• Separate the infected plants from those that show no symptoms.

How to combat botrytis in cannabis

• Remove the infected areas by pruning them with sterilised scissors. Cut at least two or three centimetres around the affected part.
• Get rid of the removed parts and make sure you sterilise the tools that you’ve used.
• Separate the affected plants from those with no symptoms.
• If you’re growing indoors, make sure you air your grow space properly after pruning, and that the humidity levels remain below 50%.
• Use organic fungicides based on manganese or copper to keep your grow at bay after pruning.
• Introduce natural enemies of botrytis, such as Trichoderma or Bacillus subtilis, which are effective for the biological control of this and other fungi that affect marijuana plants.

Identifying and treating powdery mildew, downy mildew, and botrytis effectively will be essential for your plants to reach harvest time. Remember that early detection, constant surveillance, and prevention will be your best weapons to combat these annoying intruders.