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How to Deal with Common Cannabis Plant Pathogens: Powdery Mildew

They say a good host makes a great party. Pathogens would certainly agree. If you’re overly-hospitable, these uninvited guests will gatecrash, invite their friends over, and leave your place in ruins.

In this blog series, we’re going to take a look at five of the most common pathogens which can threaten your cannabis yield, and ways to be a more ruthless bouncer. Without further ado, let’s meet our first troublemaker.

POWDERY MILDEW (Erysiphales, Podosphaera Macularis, Blumeria Graminis)

Powdery mildew (PM) can affect a whole range of crops and plants, from wheat, barley and other cereals, to onions, apples, pears and melons. Unwelcome at any event, PM will arrive at your party empty-handed, and fill its pockets with all your valuables.

What is it, and what does it look like?

A nasty and fairly common fungal disease with a penchant for destroying plants.  Powdery mildew is a symptom caused by one of the previously mentioned fungal species, with Podosphaera xanthii being the most commonly reported cause.  

As its name suggests, this nuisance is identifiable by the white or greyish-white powdery substance, similar in appearance to flour, which is easily seen on the surface of the plant. The lower leaves of the plant are most affected, but this mildew can appear on any above-ground parts of the plant.

What does it do?

One of the more recognisable cannabis pathogens, this most unwelcome of guests will move in and wreak havoc fast. It spreads both sexually and asexually, and excels at two things – destroying and spreading.

As the spores spread from the leaves and base to the flowers and buds via air currents, germinating on the leaf surface, PM will destroy a plant by suffocating the plant’s ability to obtain nutrients essential to sustenance and growth. Patches typically begin as small circles on the leaves, and as the disease progresses, look for wilting or drooping leaves. On a younger plant, leaves can appear distorted, discoloured and stunted.

Why does it happen?

Powdery mildew loves a cool, humid environment. In addition, too much water in the soil, and overcrowding, are among the chief environmental conditions conducive to the manifestation and spread of this obnoxious occupant. Poor airflow and ventilation will also increase your chances of experiencing powdery mildew, with airflow being one of the more predominant factors in allowing the mildew to settle in the first place. Aim to keep your environment at around 21-25°c, and keep humidity levels below 60%.

How do we stop it?

The good news is, there are numerous precautions we can take in order to greatly reduce the risk of powdery mildew occurring. Taking correct care of your environment is the first step to lowering the risk of this particular pathogen. Make sure you are monitoring all of these factors, and can adjust as necessary.

  • Airflow and Ventilation – Try to accommodate a minimum of two fans in the growth environment  (adjust if necessary, for larger areas). An oscillating fan, blowing air over the leaves, will make it harder for mildew to settle. A second fan, pointing out and away from the plants, will serve as a means of recirculating the air. NOTE – If you find an outbreak of PM, kill the fans immediately until the outbreak is contained and removed, otherwise the fans will spread the spores.
  • Set and monitor humidity – Preferably keep humidity in the environment below 60%. The temptation is to heat up the environment to accelerate plant growth, but this creates a welcome environment for pathogens
  • Avoid Crowding – Cramming plants together in a tight space creates the perfect breeding ground for pathogens. Leaf on leaf contact will speed up spread, and increase the humidity level in the growing area. Give your plants space, and allow them some very real breathing room
  • Prune regularly – This limits the build-up of excessive foliage around the undergrowth of the plant
  • Avoid over-watering – If soil is too damp, this will encourage pathogens.
  • Keep all equipment clean – It makes sense

The bad news is, even with the correct precautions taken, these rogue pathogens may still manifest. Checking your plants regularly and thoroughly, and knowing what to look out for, can be the difference between salvation and disaster. Take care to inspect closely, and, in the case of young plants, blistering or a strange odour on the leaves are often an early indicator of the dreaded mildew.

What to do if mildew appears

If the outbreak is small and confined to one or two areas, begin by removing the affected leaves. If this doesn’t stop the trouble, pull a gun. A spray gun, that is.

A smaller case of powdery mildew can be treated effectively using natural remedies, but commercial products such as Baby Bio and Fungus Clear Ultra are also available for treating a problem such as this. The following natural remedies can be easily obtained and are safe to use on leaves and buds:

  • Baking soda and water – one teaspoon of baking soda per gallon of water
  • Apple cider vinegar – two to three teaspoons per gallon of water

Mix in a spray bottle, then spray the solution on leaves and buds. Check the plants the following day, and repeat the process. Revisit the plants on the third day, checking thoroughly, and repeat the spraying process if necessary, but if natural remedies are unsuccessful in treating the mildew, you must immediately move to organic, commercial products in order to safely treat your crops.

PharmaSeeds plant scientist Dr Yates’ top tip in an emergency: “If nothing else is available and you need to treat the plants fast, try using a solution of around 35-45% milk in water, this has been shown on some crops to be as effective as chemical based fungicides, one spray a week keeps the mildew at bay”.  Consider this the equivalent of politely – but firmly – asking your unwanted guest to put his drink down and leave.

There’s never a good time to discover mildew on your plants, but catching it early usually gives cultivators a small window of time to treat and eliminate this common pathogen. Always keep a close eye on your plants, and environmental levels. Know what to look for, and be prepared. Set your grow area up to minimise risks, and act swiftly to eject any troublemakers from the building.

For information about other common pathogens such as grey mould, fusarium and spider mites stay tuned for the rest of this blog series (published biweekly).

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Duncan Mathers