White Powdery Mildew

Powdery Mildew

Are you observing powdery white spots developing on your leaves, new growth or stalks? Moving from one plant or group of plants to the next?

If so then you may be seeing an initial outbreak of powdery mildew, an obligate biotroph that infects and breeds in any living tissue above ground. Spores become airborne once visible spots start to form on plant tissue. Taking proper precautions and reactionary measures will reduce it from spreading and causing damage to your plants.

What is Powdery Mildew?

It is a systemic airborne fungus. It lives to eat your plants and utilize its vascular system to fuel its reproduction process, breeding and producing spores that infect any susceptible plants the spores land on.

Powdery Mildew spreads easily through spore transmission, its spores are .0001”-.0003” making them very hard to control in certain geographies and environmental conditions.

Never experienced it?

In the 4-6 days after inoculation the fungus spores will, using an enzyme, break down the epidermal wall and 


with pressure force its way into your plant to feed on nutrients contained within, spreading the infection to any living tissue.

Although this fungal parasite sounds intimidating with innovation and preventative measures any gardener, farmer or hobbyist can prevent outbreaks/transmission and in the unlikely chance that they do occur control them from causing damage or subsequent outbreaks.


What causes White Powdery Mildew? High Humidity/Temperature?

  • Powdery
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    Mildew requires a specific environment to breed and reproduce optimally.  Having extra moisture or low air flow in your room can be leading causes to an outbreak. Low temps and high humidity are also common causes. This makes prevention difficult as younger plants tend to grow better in higher humidity. Even overwatering and plant transpiration or a high nitrogen fertilizer can promote a flare up. Usually a room with high humidity and low air flow with any additional moisture is vulnerable to PM. Rooms that have Relative Humidity fluctuation between night and day cycles tend to see more PM.

  • People growing in geographic areas that tend to have high humidity can purchase a dehumidifier to help reduce RH but you must be conscious that most of these machines will add heat to your environment.


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Low/No Airflow

Powdery mildew has an easy time taking root in your room when there is low air movement and circulation. Your RH and temperature create an environment conducive to its reproduction but lack of any air movement gives it the chance it needs to settle in and reproduce.

Bad Ventilation

  • If you have PM spores circulating in your grow area and the air is never exchanged with fresh air, those spores get multiple chances to land on your plants and reproduce. This happens most often in conditions where cannabis is being grown in a closed, unventilated space – such as a closet – and precautions aren’t taken to exchange old stale air for new fresh air.
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Spores that are airborne in a grow area with limited to no air exchange have multiple outbreaks within a small space, thus increasing infection in a closed. An unventilated space, for instance a closet or sealed room, is a perfect breeding ground if precautions are not taken.

  • Any plant material that is touching or preventing airflow to create moisture pockets will have a much higher chance of PM developing. 
  • One technique used as a preventative measure is to increase air flow of a plant and environment by defoliating larger fan leaves and lower vegetation. Defoliating has a number of benefits and can be used to increase yield and create healthier plants.

How to Eliminate Powdery Mildew

Battling with PM can either be a rare or common occurrence depending on where you grow and your conditions. Taking precautions and catching it earlier will give you the best outcomes

At one time or another most growers have a run in with this prevalent fungus. Observing your plants and being aware of any spikes in RH/Temperature or environmental conditions will give you a head start at prevention. If caught early in its life cycle PM can be eradicated from your rooms and any possible damage abated.

Resourceful gardeners all over have created and used a number of homemade remedies which include:

  • Baking soda (2 tablespoons per gallon of water)
  • Neem Oil (4 teaspoons per gallon of water)
  • Hydrogen Peroxide in a 3% solution (1 teaspoon per gallon of 35% H202)

Rather than go into these methods, simply use water and a small amount of dish soap to clean off the plant and then remove any leaves with visible spores. Clean all surfaces with bleach or 6% hydrogen peroxide solution. (remember to use gloves and proper PPE)

These tips also help:

  1. Create conditions not favorable or conducive for PM to breed– Optimal control will keep Temp/RH fluctuations to a minimum. Even without direct air ventilation try to ensure air is being moved around the room with fans, either oscillating fans or fans in corners to move air in dead pockets of little to no air movement. Just enough movement to manipulate leaf’s and branches. PM will infect unhealthy plants at higher rates then healthy plants so maintaining a healthy plant will help reduce transmission of the infection. Air exchange is an important part of a plants photosynthesis process, so making sure your air is clean with a O3 generator system or a low flow inline UVC light will stop most transmissions of PM this way anywhere it lands becomes uninhabitable environment for future spores. Plus, it’s safe to use – even during flowering – and it smells awesome.
  2. Keep in mind that moving air and exchanging air is a min requirement.
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3. Treat plant with SM90 to kill spores prevent future growth – Mix 1 part SM90 to 5 parts water(I’ve found 7 parts water to be equally effective) in a clean sprayer/mister. Wait until just before your lights for off for the day and mist your (newly cleaned) plants. Get all the leaves! This diluted SM90 mixed will kill any spores it touches, and anywhere it lands becomes uninhabitable for future spores. Plus, it’s safe to use – even during flowering – and it smells awesome.

Powdery mildew can be a major pest for growers, knowing what you are dealing with will give you the advantage.  Follow these simple preventative steps, using the correct products and powdery mildew won’t have a chance!

Growing Exposed is the series that takes you behind the scenes of the cannabis industry, delivering you tips, straight from the pros, as they guide you through exclusive tours of cultivation facilities.


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