Powdery mildew is a fungal disease that can affect a variety of plant species if that plant comes into contact with spores of any fungus in the order Erysiphales. A relatively common problem for growers to deal with, powdery mildew is anything but harmless.
If left untreated, powdery mildew can damage, even kill, your high-value medicinal plants, so you must immediately begin treatment at the first sign of this fungal disease on your crops. While powdery mildew is one potential crop killer that growers face, it’s far from the only one. Read here about the 10 most common pests and diseases that plague hydroponic crops.
There are many varieties of powdery mildew, some that develop faster than others, and some that spread faster. There’s also a variety of strategies growers can utilize to combat powdery mildew, many of which are natural and pose no harm to crops or the people that consume them. If you’re producing organic crops, be sure to read here about Advanced Nutrients’ products for organic crop production.
What Causes Powdery Mildew?
Two things need to happen for powdery mildew to develop:
- Erysiphales spores need to attach to plant surfaces, such as leaves and stems.
- The temperature range needs to be between 50 and 65 degrees Fahrenheit (10 to 18 degrees Celsius), with a humidity level below 50 percent.
Powdery mildew is spread by spores, so keeping those spores out of your grow room is the first line of defense against an infestation. Grow-room cleanliness and hygiene should be a top priority, not only for reducing your risk of losing your therapeutic crops to powdery mildew, but also for preventing a variety of other problems, like downy mildew and bud rot.
Erysiphales spores can be found in a variety of climates — and they could even be lurking in your own backyard. Avoid contaminating your crops by keeping all outdoor garden tools outside and all indoor garden tools inside.
Another key to keeping your grow room clean is keeping yourself clean. Avoid going straight from your backyard to your indoor garden — your clothes carry loads of contaminants from outside. Before entering the grow room, change your clothing, remove your shoes and wash your hands to ensure that you leave mildew spores where they belong — far away from your crops.
Proper air circulation is also a must — don’t crowd plants, and be sure the room is properly ventilated. When you install a fan to circulate air through the room, attach an intake filter to catch any dust particles, spores or airborne debris that can potentially get into the room and contaminate your crops.
What Does Powdery Mildew Look Like?
Powdery mildew resembles white powder. On plant leaves, it appears as small, round clusters with a flourlike consistency. Once powdery mildew has taken hold of a plant, you’ll notice leaves become yellow and wilted.
How To Remove Powdery Mildew From Buds
So, what can you do if you find powdery mildew on your medicinal crops? Here are 12 popular treatment strategies you can use to fight this pesky garden invader.
1. UV CleanLight
UV CleanLight is a type of ultraviolet bulb that kills powdery mildew and other crop diseases, such as gray mold, allowing you to stop powdery mildew infestations without laying a finger on your crops. The use of UV light to kill powdery mildew is a relatively new crop-protection strategy that also sterilizes your entire grow room.
There are many brands of UV CleanLight on the market. These bulbs generate light at 254 nanometers (nm), a specific wavelength range of UVC energy known to kill fungi, bacteria and viruses.
However, UV CleanLight isn’t the right choice for every grower. The lights are expensive, so if you’re a small-time grower with a small-time budget, this may not be your best solution for powdery mildew prevention. Another downside to using a UV CleanLight is that prolonged exposure can put people who work the grow op at risk of retinal burn and cataracts.
2. Neem Oil
Pressed from the fruits and seeds of the neem tree, which is native to the Indian subcontinent, neem oil is a popular remedy for powdery mildew because it’s organic. Neem oil works as a natural pesticide, with growers typically adding one teaspoon to one liter of water before spraying it onto the leaves. The benefits of neem oil include:
- It’s fairly inexpensive, retailing for around $20 for a 16-ounce bottle.
- It’s all natural, so it’s a great choice for organic-crop producers.
But neem oil’s not perfect, with drawbacks including:
- Sensitivity to temperature. If it’s too cool, neem oil will solidify into a jelly-like consistency. It’s necessary to warm the water to a temperature that will prevent the neem oil from hardening on contact, then cool the mixture to a temperature appropriate for spraying.
3. Baking Soda
Also known as sodium bicarbonate, baking soda is a common remedy for powdery mildew. Its high pH makes it effective at treating and preventing powdery mildew, as it renders a plant’s surface too alkaline for the spores to thrive. The benefits of using baking soda to treat powdery mildew include:
- It’s inexpensive and available at all food stores.
- It’s easy to use. Simply mix one teaspoon of baking soda with one teaspoon of Epsom salts to create an effective treatment for powdery mildew.
A drawback to using baking soda is that it will alter the pH levels of your plants, which can in turn cause iron to become less soluble and thus less available to your crops. In this case, a plant supplement that’s rich in iron, such as Sensi Cal-Mag Xtra, can help ensure your crops receive the micronutrients they need to thrive.
4. Potassium Bicarbonate
Like baking soda, potassium bicarbonate will alter the pH levels of your crops, but it’s stronger and will kill powdery mildew quicker. An effective preventative treatment as well as a great choice for a curative remedy, simply mix three teaspoons of potassium bicarbonate with three tablespoons of vegetable oil, half a teaspoon of hand soap, and a gallon of water to create a solution that kills powdery mildew on contact.
Sulfur is a popular powdery mildew treatment because it consistently works well. When sprayed on crops, sulfur not only controls powdery mildew but also prevents new infections from occurring. The benefits of using sulfur to treat powdery mildew include:
- It’s available as a liquid or as a powder, so you can choose the one that’s easier to administer in your grow room.
- It’s natural.
But sulfur does have one big drawback: It can irritate your respiratory system and cause a burning sensation on your skin and in your eyes. Always wear gloves, breathing protection gear and protective eyewear when working with sulfur.
Some growers swear by using a dairy milk solution to remove powdery mildew from plants, citing the primary benefit of milk being cheap and readily available at any grocery store. With the milk method, growers mix 400 ml of milk with 600 ml of water then spray the solution onto the affected areas.
A solution made with garlic extract is another natural remedy for powdery mildew, as garlic contains sulfur, a potent fungicide. Commercially available garlic extracts, as well as oils pressed from garlic cloves, can be used to create an effective treatment for powdery mildew.
8. Hydrogen Peroxide
Another popular method for growers dealing with powdery mildew in their gardens is using hydrogen peroxide. One tablespoon blended with a gallon of water makes an effective solution to be sprayed onto your crops.
Hydrogen peroxide is an effective option for producers of organic crops, but you must ensure that it’s sufficiently diluted, as too much can harm or even kill your high-value plants. Avoid accidentally burning your crops by using a three percent hydrogen peroxide solution, which can be found in most grocery stores.
9. Apple Cider Vinegar
Apple cider vinegar is another readily available ingredient that can be made into a natural remedy for powdery mildew. To create this solution, mix two to three teaspoons of apple cider vinegar with one gallon of water, then spray onto leaves. Be careful to maintain this ratio, however, as too much vinegar can damage your crops.
10. Streptomyces Lydicus
This is a commercially available bacterium species that prevents Erysiphales spores from attaching to plant surfaces. Streptomyces lydicus is naturally occurring and organic.
11. Commercial Fungicides
Like sulfur, copper is effective at killing the harmful pathogens that cause powdery mildew. If you choose a commercially available copper fungicide for treating your grow op, carefully follow the directions on the label to ensure you get the ratios correct. Too much copper can harm not only your crops but also your body, so avoid inhalation when working with this type of fungicide.
Another consideration with commercial fungicides is to read the labels carefully and avoid any that contain myclobutanil. One such commercial fungicide that contains myclobutanil is Eagle 20.
Myclobutanil is effective at killing powdery mildew, which is why it’s such a popular fungicide. At low temperatures, it’s perfectly fine for humans to consume. However, when myclobutanil is heated to 400 degrees F (205 degrees C), it emits hydrogen cyanide, a compound that’s deadly to humans. If you’re growing crops that will be heated for consumption, avoid myclobutanil at all costs.
Using commercial fungicides to manage powdery mildew can hurt you when it comes to compliance testing. Acceptable pesticide levels for medicinal crops vary from state to state and country to country, certain compounds are acceptable in some markets but not others, and some markets have higher thresholds for compliant crops than others. In some places, notably Oregon, Colorado and Canada, myclobutanil is completely banned in medicinal crop cultivation, while in other markets, like California, it’s restricted but not banned.
Before you start growing, it’s important to be clear on the fungicide restrictions for therapeutic crops in your specific market. And it’s obvious but worth repeating: Steer clear of anything that can potentially harm you, even if it’s effective at killing powdery mildew. Rescuing a plant from powdery mildew isn’t worth potentially poisoning yourself or others.
12. Bacillus subtilis
Beneficial microbes are more of a powdery mildew prevention strategy than a powdery mildew treatment, but increasing the number of beneficial microbes in your garden when you notice a powdery mildew infestation can certainly bolster your treatment efforts. Two Advanced Nutrients products that contain Bacillus subtilis, a specific strain of beneficial microbes known to boost crop immunity, are Voodoo Juice and Tarantula.
When Bacillus subtilis is present in your crops’ rhizosphere, it helps the plants fight off disease-causing fungi in two ways:
- It induces the plants’ natural systemic resistance.
- It physically coats the plants’ roots to stop fungi (and harmful bacteria and viruses) from penetrating the roots’ skin and infecting your crops.
Compost Tea As A Way To Introduce Beneficial Microbes To Your Crop
Many growers prevent powdery mildew with compost tea, which introduces beneficial microbes into their crops’ rhizospheres. Growers can make their own compost tea or purchase concentrated compost teas that are ready to dilute and use immediately. The benefits of compost tea include:
- It can be made organically.
- You can make your own compost tea.
The drawbacks to choosing compost tea as your powdery mildew prevention strategy include:
- It takes a long time to brew an effective compost tea.
- Compost tea has a very unpleasant odor.
Making your own compost tea is a fairly simple process. Start with high-quality compost. Some growers choose to pretreat the compost by blending it with humic acid or fish hydrolase and leaving the mixture to stew for two to three days. This is not a necessary step, but one that many growers feel makes the compost more effective.
Recipe For Compost Tea
The next steps for brewing a high-quality compost tea are as follows:
- Fill a bucket 1/3 full with compost
- Fill the remainder of the bucket with unchlorinated water
- Leave the mixture to steep for three or four days, stirring it occasionally
- Strain the mixture into another bucket through a porous fabric like cheesecloth
- Dilute the liquid with more unchlorinated water until it’s the color of weak tea
Rather than mixing water with loose compost, some growers choose to put their compost into a nylon stocking or mesh bag and submerge it in the water, like they’re steeping a giant tea bag. Whether you choose this method or the method outlined above is up to personal preference.
To speed up the process, some growers opt to add airstones to their brewing buckets to agitate and oxygenate the tea. Compost tea made with this method takes only 24 to 36 hours to brew, rather than a few days.
So, what is the best method for dealing with powdery mildew in your grow op? The short answer is that there really isn’t a “best” treatment. It comes down to what you can do most effectively within your budget and resources.
Powdery Mildew Prevention For Therapeutic Plants
If you’re dealing with powdery mildew in your garden, the 12 strategies listed can be effective options for killing the infestation without killing your high-value crops.
But there’s an even better way to avoid losing your therapeutic crops to powdery mildew: Prevent it from occurring in the first place.
More tips for preventing powdery mildew include:
- Don’t crowd your crops: When plants are placed too close together in a grow op, air cannot effectively circulate among them. This will drive up the humidity level in the room and create an environment in which powdery mildew can thrive.
- Keep your indoor garden properly lit: The fungal pathogens that cause powdery mildew to flourish need a damp, dark environment, so any plants that don’t get sufficient light are at a higher risk of infection than those in brighter areas. If there are stems and leaves growing on lower portions of your plants that just can’t get the amount of light they need because of their positioning, consider pruning them to eliminate the chance of allowing powdery mildew spawn sites to spread.
- Make beneficial microbes part of your grow strategy: Microbes from the bacterium strain Bacillus subtilis can boost your crop’s immunity to pests and pathogens by inducing their natural defense mechanisms. You can find Bacillus subtilis in Advanced Nutrients products like Voodoo Juice and Tarantula.
Enhancing the natural defense systems of your crops isn’t the only advantage to gain from adding beneficial microbes to your grow. Read here about all the ways beneficial microbes enable your crops to grow stronger, more efficiently and resistant to pathogens.
Even when you prioritize powdery mildew prevention in your garden, it’s still possible for spores to get into your grow and breed on your crops. After prevention, your best line of defense is prompt action to get rid of the mildew. When you submit your medicinal crops for purity testing, any traces of powdery mildew will cause them to fail. As a grower cultivating a therapeutic product, the safety of customers needs to be the top priority.
If you see signs that this pest, or any other disease that can harm your crops, has set up shop on the leaves of your therapeutic plants, the Advanced Nutrients Grower Support hotline can help you diagnose and treat the problem. Don’t wait until it’s too late to get the answers you need to protect your high-value crops — act now!
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