Vibrant green stems and leaves. Brown buds are covered with glistening trichomes. Purple, pink, yellow, red, orange, or blue shades often take the place of brown in mature flowers.
We associate many colors with the cannabis plant, one of nature’s many beautiful wonders.
But when you look at a weed plant and see white patches appearing on the universally-recognized green fan leaves — you have a big problem.
The problem has a name: powdery mildew. It can affect many types of plants, but it’s a major threat to marijuana crops.
Here’s the good news: powdery mildew probably isn’t going to kill your plants if you’re paying attention.
Here’s the bad news: it can render your bud unusable.
Growers need to treat powdery mildew as soon as they see the first signs. Here’s what you need to know and how to do it.
What Is Powdery Mildew?
Powdery mildew (often called white powdery mildew) is a plant disease caused by fungi. More specifically, it’s caused by “obligate biotrophic fungal pathogens.”  
That’s a long name used to describe a number of fungi that need living hosts to survive; they stay on plants and feed on the nutrients they extract. Rust fungus, another potential hazard to cannabis plants, is caused by the same type of fungi. 
In other words, powdery mildew doesn’t immediately kill plants outright. The fungal disease slowly drains their life while spreading, and eventually overtakes the entire plant as the leaves develop yellow spots which then turn brown. At that point, the plant is lost.
Powdery mildew is usually carried by the wind or insects until it finds an appropriate host, and there are many hosts to choose from. The fungi can affect most plants, but they seem to love lilacs, roses, cucumbers, squash, zinnias — and cannabis. 
Certain weather conditions make it more likely that a grower will have to deal with these fungi. The disease proliferates in spring and fall conditions when temperatures are between 70-80 degrees and humidity is high at night.
That doesn’t mean indoor growers should breathe easy. The most important conditions that encourage the growth of powdery mildew are moderate temperatures and high humidity, which can put indoor crops at risk.
The first step for growers is to look for and recognize powdery mildew. The second is to take immediate action. Let’s take those one at a time.
What Powdery Mildew on Cannabis Plants Looks Like
The disease is accurately named.
Powdery mildew usually looks like white powder building up on your plant’s leaves, although it can also look more gray than white. Fortunately, it doesn’t take much effort to see the powder.
It usually starts growing toward the bottom of the plant on the underside of cannabis leaves and then works its way toward the top over time. As it does, it consumes more and more of the plant’s nutrients.
Mold spores may simply sit on leaves for a while, waiting for the right environmental conditions. When powdery mildew begins to grow, though, the white-on-green contrast makes it easy to see.
The disease first appears as small white spots or patches; the powder will have the consistency of flour. If you haven’t noticed the spots or patches on the leaves’ undersides, you’ll see there’s a problem when leaves begin to wilt.
Experts always advise growers to regularly inspect their plants for signs of disease or pests. Growers often ignore that advice — at least, the “regularly” part.
The existence of powdery mildew and similar fungus-caused disease, however, is a very good reason to pay attention to that expert advice. Look daily, if you can, for the signs of white powdery mildew. It’s fairly easy to get rid of if you discover it early enough.
How to Treat Powdery Mildew on Cannabis
A careful, daily inspection of cannabis plants will reveal the existence of powdery mildew while the plants are still salvageable.
If you see the beginnings of a powdery mildew infestation, simply cut off the affected leaves — and gently place them into a bag for disposal. Be sure they don’t come in contact with any other leaves or plants because the mold spores will hop aboard. Also, be sure to fully disinfect the scissors or knife you’ve used before bringing the tools back into your grow.
However, if you’re like most of us, you check your plants once or twice per week and don’t often look at the undersides of the leaves. That means powdery mildew can get the upper hand and begin its spread before you realize there’s a problem.
When you’re in that situation, here are your options.
- Horticulture oil: Neem oil, vegetable oils, or petroleum oils (available at garden shops) can often effectively fight powdery mildew, as well as other fungi and insects. Use it in moderation because it can burn leaves.
- Milk spray: Spray the plants (not just the affected leaves) with a mixture of 25% milk, 75% water. Believe it or not, milk protein reacts with sunlight (or grow light) and provides an antiseptic effect.
- Baking soda: Mix ½ teaspoon of baking soda (or a tablespoon of hydrogen peroxide) with a quarter of water and spray the plants.
- Sulfur or copper solutions (also available commercially) will work, but they can burn the person applying them as well as the plants, and will damage plants if the temperature is above 90°.
- Chemical fungicides: If you’re not growing organically, there are a number of choices. Some of the most effective fungicides use chlorothalonil or Thiaphanate-methyl as active ingredients.
Most of these approaches can help fight other types of infestations, but one warning: mildew and insects quickly develop immunity. Mix up the treatments you use so you don’t run into this issue.
Can You Prevent Powdery Mildew?
No, but you can make conditions as inhospitable as possible.
- If possible, keep the relative humidity in the grow room below 55%. As we’ve noted, these fungi love humidity.
- Ensure good airflow and ventilation in the grow room, or if you’re growing outdoors, give plants plenty of space to grow. Strong airflow makes it difficult for mold spores to settle on plants.
- Trim the plants discriminately. Moisture — and humidity — builds up when leaves are touching each other or forming a canopy.
If you take one thing away from this article, though, it should be this: make the effort to check your plants daily. You won’t regret spending a few extra minutes, and it will give you more time to appreciate the beauty of your grow and anticipate the dank crop that you’ll be harvesting.
Powdery Mildew on Cannabis: FAQ
Q: Is there anything you can use to prevent the growth of powdery mildew?
A: Some people have had success spraying their plants with horticultural oil or milk spray before they have a fungus or insect problem — or at least they say they have. It’s hard to prove that a preventative treatment worked, but it seems to be worth a try.
Q: Is there a point when the only alternative is to throw the plant away?
A: If powdery mildew gets into the flower, it can’t be consumed. And if you find that you can’t get the mildew under control and the plant has turned brown, it’s pretty much a lost cause. Just be sure to carefully remove the plant from the grow so it can’t pass the mold on to the rest of your crop.