Also known as white mould or Oldium, powdery mildew is a fairly common problem faced by many growers. This mould is a parasitic fungus that lives on living plant tissue and can spread to leaves and buds. While powdery mildew is unlikely to outright kill a plant, you should know that infected flowers are not suitable for smoking.
- What causes powdery mildew on weed plants?
- What are the symptoms of powdery mildew?
- What are the effects of powdery mildew?
- How to treat powdery mildew on cannabis plants during vegetation
- How to treat powdery mildew on flowering plants
- How to prevent powdery mildew
- Can I smoke weed with powdery mildew?
- You gotta fight the powder
Once powdery mildew sets in on a weed plant, it will begin to spread rapidly, particularly in areas with poor air circulation and/or high humidity. Growers must regularly inspect their plants and maintain an environment that isn’t suitable for fungal growth. Preventive measures include proper spacing of plants, ensuring good air movement, and using fungicides when necessary. Early detection and treatment are key to managing powdery mildew and preventing it from ruining your crop.
What causes powdery mildew on weed plants?
The most common causes of white powder appearing on weed plants are high humidity and poor air circulation. As a fungus, powdery mildew thrives in these conditions; specifically with humidity exceeding 60% and temperatures from 15°C to 27°C. Additionally, overwatering and overcrowding can create a favourable environment for the fungus.
A lack of proper light and excessive nitrogen in fertilisers can also contribute to the spread of powdery mildew. Cannabis plants that do not receive adequate light are particularly susceptible, as are those that experience sudden temperature fluctuations.
What are the symptoms of powdery mildew?
The most noticeable sign of powdery mildew is a flour-like white powder appearing on the tops of younger leaves, though it can also start on older growth. This powder typically starts as small round white patches on the leaves and stems. While it typically appears on the tops of leaves, it can also show up on the undersides. If left untreated, the damaged leaves will turn yellow, deform, bend, or even break off.
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What are the effects of powdery mildew?
If left untreated, powdery mildew will spread across your entire plant or garden. While rarely fatal on its own, it can have several knock-on effects for a weed plant.
- Reduced photosynthesis
The white, powdery covering on leaves makes it harder for the plant to turn light into food, leading to reduced growth.
- Leaf damage
Infected leaves may turn yellow, curl, and eventually fall off, further limiting the plant’s ability to sustain itself.
- Nutrient drain
Powdery mildew competes with the plant for nutrients, weakening the plant over time.
- Secondary infections
Weakened plants are more susceptible to other diseases and pests, which can further damage or kill the plant.
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How to treat powdery mildew on cannabis plants during vegetation
If you notice white powder on your weed plants during veg, don’t wait to see if it spreads. If it’s there, it will get worse and fixing it will get harder, take longer, and cause more damage. Fortunately, treating it is fairly straightforward.
1. Manually remove the mildew
The first and most important thing to do is to remove as much of the mildew as you can. Simply get a wet paper towel and gently wipe off every affected leaf. This can take some time and it’s important to be gentle. Once you’ve removed what you can, it’s time to move to the next step.
2. Potassium bicarbonate
A natural fungicide, potassium bicarbonate works by creating a highly alkaline layer on the plant’s surface. This layer actively penetrates the cell wall of the mould spores preventing them from developing further or reproducing.
To use potassium bicarbonate to kill powdery mildew, simply mix approximately 10 grams into one litre of water and add in a few drops of vegetable oil or mild dish soap (this helps the solution stick to the leaves). Next, simply spray this mixture lightly and evenly across any affected areas of your cannabis plant.
3. Baking soda
Less effective but more readily available than potassium bicarbonate, baking soda (sodium bicarbonate) functions in the same manner. Simply mix about 15 grams of baking soda with a litre of water and a few drops of mild dish soap or vegetable oil and spray the affected areas.
4. Milk
Another all-natural solution for combatting powdery mildew is milk. While it might seem like an odd choice (who wants to leave milk in the sun?) there’s some very solid science at work here. While milk is somewhat acidic (pH of around 6.5), when it comes into contact with living plant matter (especially when exposed to sunlight), its pH can become far more alkaline making it inhospitable for fungus. Basically, there are several proteins in milk (e.g., casein) which can bind to a plant’s surface and raise the solution’s pH.
On top of this, milk triggers the plant’s immune response making it more capable of fighting off infections. These proteins, when exposed to sunlight, undergo a photochemical reaction that produces compounds with fungicidal properties.
To use milk, simply create a mixture that is one part milk and nine parts water (1:9), mix well, and spray directly onto the affected surfaces under direct light. Reapply this mixture once a week for the next few weeks or until the mildew is completely gone.
5. Neem oil
Neem oil is one the most commonly used (and most versatile) treatments favoured by growers. This natural oil contains several compounds that work as effective antifungal and antimicrobial agents. Additionally, neem oil stimulates a plant’s immune system in a variety of ways.
To use neem oil, simply mix the oil with water according to the package’s instructions—typically 5-10 ml of oil per litre of water—as well as a few drops of vegetable oil or mild dish soap. Then, all you need to do is spray the leaves lightly and evenly. Repeat this every 7-14 days until the mildew is gone.
Neem oil is a great choice as both a cure and a preventative for almost anything that lives on your cannabis plants from powdery mildew to spider mites. Check out our in-depth guide on dealing with pests to learn how to keep your plant healthy.
6. Fungicides
In the most extreme cases, chemical fungicides may be required. However, it’s typically better to scrap the plant and start over if the infestation has reached this point. Many of these mixtures contain large amounts of sulfur which is likely to affect the taste and smell of any flowers that you may grow. Furthermore, these chemicals are incredibly harsh and can (in large quantities) damage your weed plant.
How to treat powdery mildew on flowering plants
Anything that is sprayed onto buds will get absorbed, trapped inside, and can take weeks or months to go away. If you’re noticing white powder before flowering, it’s usually a good idea to start treating it before switching to a 12/12 light cycle.
If powdery mildew only begins to appear mid-flowering, you do have options. Of the above options, potassium bicarbonate is the safest as it contains the least harsh chemicals. No matter what you use, never spray the flowers directly. While powdery mildew can live in flowers, it typically starts on leaves, so treating it early is paramount.
If you begin to see it growing on or near flowers, use a cotton swab or sterile gauze to lightly dab any surrounding leaves with your treatment of choice being very careful to avoid theflowers if at all possible. No matter what treatment you choose, do not apply it in the final 2-3 weeks of flowering.
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How to prevent powdery mildew
While all of these treatments can also be used as preventatives, several steps should be taken to help prevent (and treat) powdery mildew in your garden.
1. Control humidity
As a fungus, powdery mildew thrives in humid conditions. The ideal humidity for cannabis plants is around 60-65% during vegetation and as low as 40% during flowering. As mentioned, powdery mildew is most common in areas with humidity above 60% so by keeping it lower you’re both helping your weed plants and preventing mould. A simple dehumidifier is a great choice for almost any grow room.
2. Keep a clean grow room
Always thoroughly clean your grow space before and after every grow. Isopropyl alcohol and hydrogen peroxide are both great choices for sterilising your space and any tools that you may use; always clean yourself and your tools before working on your cannabis plants.
It’s also important to regularly remove any fallen leaves or dead growth to prevent the spread of any fungal growth. Once a leaf has been damaged, it’s not going to recover. Remove any damaged or dead leaves immediately to minimise the spread of any pathogens.
3. Give your plants space
Powdery mildew commonly forms where leaves overlap as these areas tend to have reduced airflow. While some overlap is usually unavoidable, taking steps to tie down branches and keeping them from getting too close is always a good idea.
4. UVC lights
UVC lights emit ultraviolet light in the so-called “C band” which has a much shorter wavelength than UVA or UVB light. This short wavelength gives UVC light unique antimicrobial properties and it has been seeing increased use in a range of sterile environments from operating rooms to food processing.
These lights aren’t meant to be additional lighting in your grow space; in fact, too much UVC light can severely damage your weed plants. Instead, it should be used as part of routine plant maintenance or as spot treatment for things like powdery mildew. Always use gloves and goggles when handling UVC lights and consult the manufacturer’s safety guidelines.
With a simple handheld UVC light, it’s possible to completely disinfect a surface in a matter of minutes. Note that this should be used in combination with other treatments to make sure that nothing is left behind.
5. Air filters
Any fan or ventilation risks spreading powdery mildew spores so taking a few precautions (especially on your ventilation fans) can make a huge difference. HEPA filters are a simple and cost-effective choice for any grow space and will drastically reduce your cannabis plants’ exposure to many airborne diseases.
Can I smoke weed with powdery mildew?
If you’re seeing powdery mildew on dried buds, throw them out. It is NOT recommended to smoke any flowers that have mildew on them. While the extract strain of fungus that causes this disease isn’t (directly) harmful to humans, inhaling fungal spores can lead to a range of respiratory problems. It’s also worth noting that weed can even grow powdery mildew once it’s been cured if it’s improperly stored, so always be checking your weed for mouldy signs and smells.
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You gotta fight the powder
Powdery mildew is one of those problems that any grower will encounter sooner or later. If left untreated, it can completely ruin a harvest but a few simple steps are all it takes to keep your cannabis plants healthy. Hopefully you found this article in time to nip the powdery mildew in the bud! Do you have any questions or tips? Share them below!
