Powdery Mildew, or PM for short, is a fungal disease that causes the growth of mycelium (fungal strands/threads) on the external surface of leaves, stems, fruits and flowers. PM can be a dreadful foe but an old saying goes “An ounce of prevention is worth a pound of cure.” A lesson you will almost certainly learn when cultivating cannabis. So here’s our guide on some products and methods for powdery mildew prevention.
Mildew Infection
Why is powdery mildew bad?
Mildews feed on living plant tissue penetrating cells to sap food and fluids. This affects growth and in the end, yield. If left untreated, plants can easily succumb to the fungal infection.
Consuming flower tainted with powdery mildew can be very harmful to your health. Inhalation can cause serious irritations, coughing and infections. People with respiratory problems could potentially have more severe reactions and infections.
How do mildew infections occur?
Fungi spores are all around us, the atmosphere is full of them. Powdery mildew will spread thousands of tiny spores when ever disturbed. If spores land on a favored surface where conditions are right, infection can occur.
PM likes warm, humid and moist conditions where it thrives. Ideal temps are around 15-27°C/60-80°F, and believe it or not, it often does well in dry climates too. However, powdery mildew does require fairly high relative humidity to spread. Like rain or when condensation between overlapped leaves occurs.
For indoor growers, poor ventilation, air circulation and temp fluctuations can aid powdery mildew in taking hold.
How is powdery mildew prevented?
The best defense against PM is a healthy plant, making an infection difficult for the fungus. Still, when conditions are right, PM spores can land on a surface taking hold. Once infection occurs it can be very difficult to get rid of. By completing prevention methods on the regular you can fend off infection in the first place. This will form part of your Integrated Pest Management (IPM) program. Should you not have one, it’s a good time to start for powdery mildew prevention.
It’s also important to note whether a fungicide is safe to use in the flowering stage of cannabis. Certain products as well as systemic fungicides for example are not safe for flowering cannabis and human consumption.
Methods for Powdery Mildew Prevention
Oils
With regular use, neem oil has proven to be very effective in preventing powdery mildew. Neem does have a strong smell and can leave a residue and flavour on your finished product affecting taste. Use neem in the veg stage of your grow as prevention for mildew and as insect repellant. Oils also act as a wetting agent, preventing liquids from beading and rolling off the leaf surface. This ensures proper coverage of your foliar spray.
Bicarbonate of Soda & Non Detergent Soap
Baking soda and dish wash liquid is a common household recipe. Used in gardens across the world and often quite effective. Most recipe’s instruct to dissolve a half teaspoon of bicarbonate of soda in 1 liter of water with a quarter teaspoon of non-detergent soap. Generally, plants don’t like soap so keep detergents, degreasers and bleaches well away from them. The alkali nature of the solution will also help raise the pH of the leaf surface, something mildew doesn’t like. To give you a heads up, we got a salty taste when we smoked product treated with bicarb, so use it in early flower only.
Potassium Bicarbonate Organic Fungicide
A Potassium Bicarbonate solution will kill PM colonies and spores on contact. Most likely better than what baking soda would. Dissolve one teaspoon in 1 liter of water as a widely accepted organic fungicide. Do not apply after week 4 of flowering cannabis.
Bio Fungicides
There are biological controls for PM and one of the most effective is Ampelomyces quisqualis. A fungi that is a hyperparasite to PM. After application, for approximately 7-10 days, the mycoparasite spreads within the body of the mildew colony. Although still alive, reproduction of the mildew stops during this time. Thereafter, the AQ-SF starts to fruit inside the PM, killing it in 2-4 days. If needed, apply AQ-SF on crops up to 3 days prior to harvest as it poses no threat to humans. It’s most effective when used regularly in an IPM program. Check compatibility of IPM products before applying AQ-SF. This is our personal go to product for powdery mildew control. You can extend the some what short shelf life of biological products like this by keeping them in the fridge.
Jadam & KNF Fungicides
The Korean Natural Farming techniques, or KNF for short, and Jadam techniques are increasing in popularity among cannabis growers. Methods are provided to create your own organic inputs and pest management solutions for cheap. Or simply if you prefer doing it your self. Wetting agents, sulfur and ferments are used to concoct mixtures aiding in your fight against powdery mildew. The downside is that they can take time to make. Neudosan is a commercially produced soft soap and wetting agent.
Copper & Sulphur Fungicides:
Proven to be a very effective fungicide, copper will help in the prevention and treatment of most fungal infections. Biogrow’s Copper Soap is safe to use in the veg stages of cannabis growth. Do not apply to flowering plants. After application, the fungicide will break down into fatty acids and copper over a day or two, both available for plants and microbes to use.
Like copper, sulphur is also an effective fungicide. A wettable version is applied for the last time in the early stages of flower. Sulphur can also aid in the prevention and control of certain pests like thrips, however it will also kill beneficial insects on contact. Sulphur is not compatible with oils like neem and canola.
Many gardeners choose not to use broad spectrum fungicides. Any excess run off onto soil will also kill fungal life in the growing medium.
Hydrogen Peroxide
A clean, versatile compound that is a strong oxidizing agent and disinfectant. Peroxide for short, is basically water with an extra oxygen molecule. Peroxide solutions can be effective in killing PM colonies and spores. Mix a 3% solution with 4 parts water and spray on affected plants in the coolest part of the day. Hydrogen peroxide has the potential to damage plants, burn pistils and oxidize trichomes with foliar applications so beware. Handle it with extreme care and read more on hydrogen peroxide here.
Milk
It sounds crazy but Brazilian agricultural researcher Wagner Bettiol successfully controlled powdery mildew on zucchini grown in a green house with milk. Publishing his research in 1999, he found that high concentrations of milk were often more effective than the conventional fungicides tested. Bettiol sprayed plants with milk at 5, 10, 20, 30, 40, and 50% solutions, either once or twice a week. Whats very important to note is that the milk solution needs 5 hours of sunlight after application to be effective. Proteins in the milk interact with the sun’s rays giving it it’s anti fungal properties. A word of warning, milk can smell terrible when it breaks down, so not advisable for indoor growers or when close to harvesting buds. Although effective, it’s not something we recommend.
Silica
Silica can be important to plants in creating healthy cell walls. The stronger the plant, the more difficult it is for mildew to penetrate it’s cells. Silica is quite effective in strengthening the plant when applied as a foliar during the veg stage. The last application during the flowering stage would be around week 2 or 3. Do not mix Silica with other calcium nutrients or products. Apply it on its own with a compatible wetting agent. PlantGuard, mono silicic acid, is our preferred silica product for powdery mildew prevention.
Integrated Pest Management (IPM) Program for Powdery Mildew
A good management program can curb problems during your grow if followed properly and aid in powdery mildew prevention. To improve success rates, it’s possible and often advisable to use multiple products together. Skipping a spray is all it takes to let things get out of control and ruin a harvest. Whats also important is to research and check compatibility with the products you plan to use.
Here are a few recipes we’ve found to be effective for powdery mildew prevention:
Vegetation Stage – Per Liter Water
10ml Bioneem
5ml Copper Soap
Flowering Stage
Option 1 – Per Liter Water (Up to the 2nd last week of flower)
AQ-SF – 5ml
Bio-Tricho – 5ml (Trichoderma slows the growth of many plant pathogens)
Canola Oil – 5ml (Wetting agent, use up to week 6)
Option 2 – Per Liter Water
Hydrogen Peroxide 50% – 5ml
Canola Oil – 5-10ml
Applying Foliar IPM Solutions
Best practice is to apply the foliar sprays at sunset or just before lights off. If indoors, switch off all fans leaving your extraction running. Apply to the stems starting from the bottom up following the main stem and then each side branch too. Turn the plant 180 degrees and repeat. Next, spray the undersides of the leaves from the bottom up while rotating the plant a little at a time until you’ve gone right round from top to bottom. Move onto spraying the upper surface of the leaves from the top down also rotating the plant and finally from the side too turning as you go. By this time the plant should be dripping with solution. Any spots missed is a place mildew can grow so make sure you do a good job.
Conclusion In Preventing Mildew
We often get calls with customers who have issues with powdery mildew in late stages of flower looking for a cure. When we ask what they were doing as prevention there’s usually just silence on the other side. Our advice is to get an IPM program together, chat to other growers about what they use if you need some help. The worst thing you could do is nothing and expect for everything to go without a hitch. Rather do something and have peace of mind that your cannabis flower is clean and safe to smoke. It takes a bit of effort for successful powdery mildew prevention but it’s well worth it.
More Tips
Growing strains resistant to powdery mildew can also aid in prevention. Although there isn’t one single cannabis strain that is totally immune to the disease.
Replace air in the grow room every 2 minutes. This is the minimum requirements for your plants. Adding oscillating fans will improve circulation. Keeping temps constant without big fluctuations will also curb infection.
Keep your grow room clean and deep clean after every harvest. Clean with a hydrogen peroxide solution if possible. Wipe down all your equipment inside the room like fans and wash out trays and pots. Rinse dehumidifier and airconditioner piping and tanks.
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