Powdery Mildew on Cannabis: What It Is and How to Get Rid of It

Powdery mildew is one of the most common fungal infections in a cannabis grow, and it shows up as a white, flour-like powder coating the leaves and buds. You keep it out with steady temperatures, controlled humidity and clean airflow, and you fight it by pulling infected leaves and treating the plant with sulfur, potassium bicarbonate, UV lamps or fungicides, depending on the stage.

It's not the deadliest fungus out there, but if you don't treat it early it can spread through your whole grow, damaging the plants and leaving the buds unfit to smoke.

Once an infection gets out of hand, it blankets the leaves, stems and the buds in white, causing yellow leaves, leaf drop and eventually a dead plant.

How to identify powdery mildew on weed plants

Powdery mildew weakens plants, makes buds unsmokable, can wreck an entire harvest and, worst of all, can be seriously bad for the health of anyone who smokes contaminated weed.

It's pretty easy to recognize, since the spores are white and you'll see leaves that look dusted in white, and if you catch it early it's treatable. What trips growers up is mistaking it for resin. So here's everything you need to identify it correctly:

• By the time you can see powdery mildew on a leaf with the naked eye, there are already hundreds or thousands of spores there. A few isolated spores are only visible with a loupe or a microscope.
• As the spores spread, they form faint whitish patches on the leaves that look like ash, dust or flour. If you don't know what you're looking at, a moldy bud is easy to confuse with plain dirt from outside. The patches sit on the top side of the leaves, the side facing the light. That detail matters, because it's how you tell powdery mildew apart from downy mildew, a similar fungus that shows up on the underside of the leaf.

Prevention always beats cure. A clean grow room with good ventilation, controlled humidity and temperature, plus a daily look at your plants, goes a long way toward keeping powdery mildew out.

There are three moments when you're most likely to see this kind of mold on cannabis: during the vegetative stage, in the last weeks of flowering, and during drying and curing. Powdery mildew thrives on crowded, overwatered plants and in high-humidity environments, which is why clean airflow is so important.

How to prevent powdery mildew on cannabis plants:

Avoid big temperature swings

The ideal temperature range for growing cannabis is between 20 °C and 29 °C (68–84 °F), but the tighter you keep the swings, the less vulnerable your grow is to powdery mildew. When the lights go off, temperatures can nosedive for a short stretch. As the air cools, it holds less water, so that moisture gets released into the room. That can lead to micro-condensation and high humidity, setting the stage for spores to germinate and for mildew to show up on drying buds.

Manage leaf-surface pH

Wiping powdery mildew out of your buds and the environment completely may be impossible, but you can protect the leaves if you catch it in time. Whether the white fungus on your buds manages to spread through the grow depends heavily on the pH of the leaf surface, so nudging that value up or down is key.

Products containing potassium bicarbonate can temporarily raise leaf-surface pH, while sulfur-based products lower it. Ideally, don't use them during flowering; use them preventively in veg to keep mildew off your buds later.

Filter the problem out

Moldy weed can contaminate the entire grow if you don't fix one of the biggest culprits: poor ventilation. That said, if mildew is already present and you crank the fans, you can blow spores onto every other plant in the room. That's why air filters are a smart preventive move. Installing air filters can help trap spores from infected buds and keep them from getting pulled back into the grow space.

If the fungus has already hit the leaves, remove every leaf that shows mildew and take it well away from the grow room. Be careful not to let infected leaves brush against any other part of the grow. Remember, fungal spores travel and spread easily. Propolis and horsetail are two great natural allies when it comes to getting rid of powdery mildew.

Important: drying is one of the riskiest times for a mildew infection, and it's also the most painful one for a grower. That's why it's key to learn how to cut, dry and cure your harvest properly. 

How to get rid of powdery mildew during the vegetative stage

If you spot powdery mildew while your plants are still in veg, you can combine the following treatments:

Anti-fungal UV lamps

If mildew is clearly present on your buds, you can use anti-fungal lamps like Clean Light. Run the lamp over the plants once a day from about 20 cm (8 in) away. Always wear protective glasses and gloves.

Sulfur

Another way to clear powdery mildew from buds is a sulfur product such as Elosal. It's sold as a fully organic product, so it can be used in both veg and flower. It's applied as a foliar spray, meaning you soak the whole plant. Watch the temperature: above 35 °C (95 °F), sulfur can become phytotoxic and burn your plants.

Chemical fungicides

During veg you can also use a chemical fungicide with both contact and systemic action.

Pull the infected leaves

If the plant isn't badly affected but a few leaves are covered in mildew, remove them. Do it gently so you don't shake the leaf and scatter spores. Then put them in a bag, seal it tight and take it out of the grow.

Get rid of the plant

If you've got several plants and one is completely taken over by powdery mildew, it's better to lose one plant and save the rest than to try to rescue it and end up infecting everything else. If you're going to toss it, cover it completely with a bag and seal it first so you don't spread spores. Keep the container and clean it.

Wash your hands and change your clothes

After handling plants with mildew on the leaves, wash your hands well, rub them down with alcohol and change your clothes before touching healthy plants.

How to treat powdery mildew during flowering

Powdery mildew in flower is bad news, because it can reach the buds and your yield will always take a hit.

In late flower, treat it with biological fungicides to stop the spread as fast as possible. Some of the most effective ways to knock out powdery mildew in late flower:

• Biological fungicide: applied as a foliar spray every 3–5 days, depending on how far the fungus has spread.
• Anti-fungal lamps: during this stage, use the lamp every day at a distance of 20 cm (8 in) above the plant.
• Remove the plant: follow the steps we covered above.
• Apply Oidioprot: it kills the mildew on the plant, and the lamp makes sure every last spore is cleaned up.

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