Downy Mildew and How It Acts Fast
                     Photo by Nick Harsell on Unsplash
Downy Mildew and How It Acts Fast
When the growing conditions are not just right, it can wreak havoc on your cannabis crop. There are dozens of viruses, viroids, 
molds, and pests that can cause crop loss, and understanding each one is important. Downy mildew is one of the most 
common problems growers will face in their careers.
When people think of mildew, they often picture powdery mildew. While this type of mildew is caused by similar factors, both 
powdery mildew and downy mildew are two separate blights. By comparison, downy mildew is discussed far less frequently 
even though it is just as problematic. Let’s take a closer look at what downy mildew is, what causes it, and what growers can do 
to stop it.
What is Downy Mildew?
Downy mildew is caused by Pseudoperonospora Cubensis, a fungus-like organism that is traditionally known to impact crops 
like squash and melons. However, downy mildew is also known to impact cannabis plants if growing conditions are not 
maintained. It can be confused with botrytis, a mold that is a well-known blight that nearly every grower has dealt with at one 
point or another. However, downy mildew and botrytis are two separate things.
Cannabis crops can become infected with downy mildew by being exposed to extended high humidity. This can be caused naturally 
or artificially depending on the grow setup. Neither outdoor nor indoor grows are immune from downy mildew, though it is 
arguably easier to lessen your chance of downy mildew in an indoor setup.
Unlike powdery mildew, downy mildew will not appear as a powdered sugar-looking substance. Both types of mildew are caused by 
humidity and spread quickly throughout the crop. However, downy mildew can mimic nutrient deficiencies or excesses. Therefore, it 
is vital to know how much of which nutrient you are feeding to your plants.
How Does Downy Mildew Affect Cannabis Plants?
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While downy mildew can impact traditional crops like melons and squash, it can also have a big impact on cannabis 
crops. Typically, downy mildew can be spotted on the leaves of the plant, usually in varying shades of yellow, light 
green, and brown. These spots can be seen within the canopy where the humidity is highest, usually on older leaves. 
However, this type of mildew can spread throughout newer growth and into less dense canopy if left untreated.
Similar to powdery mildew, downy mildew can impact the overall yield of a cannabis crop. An infected plant will place more resources 
towards repairing damaged leaves and stems rather than into the growing bud. The best practice is to treat downy mildew, then 
enforce preventative measures to avoid a repeat infection.
How to Combat and Prevent Downy Mildew from Affecting Cannabis Crops
Downy mildew may sound like a death sentence for your plants, but with a bit of preparation and work, you can protect your cannabis 
crop. Cannabis cultivation is all about maintaining a balance of everything your crop needs to reach its full potential.
Keeping an appropriate temperature and humidity balance is one of the most important things to do in order to keep downy mildew 
away from your plants. This is dependent on the climate of the area the grow is in, what stage of their life cycle the plants are 
currently in, and sometimes even the weather outside can play a role. Temperature and humidity balances can be maintained with 
equipment and careful planning.
The second thing to do to keep downy mildew out of your plants is to have good airflow throughout the room. This is often 
accomplished by using rows of oscillating fans in an indoor setting. Air flow will help keep moisture from getting trapped within the 
canopy and from raising the humidity. As an added bonus, it will also help the plants develop stronger stems and root systems 
when compared with no or little airflow.
Finally, keeping your cannabis plants defoliated can help that airflow do what it needs to do and keep downy mildew from making 
a home in the canopy of your cannabis crop. In addition to having good airflow throughout your grow rooms, defoliation can also 
help open up the canopy and prevent humidity from rising. This form of plant maintenance serves a variety of purposes, with 
mildew prevention being just one of them.
However, if you happen to find downy mildew in your cannabis crop, there are measures that can be taken. Depending on where 
your crop is in its lifecycle, it may be possible to use certain fungicides to combat the mildew. However, the products that can be 
used on cannabis plants are regulated by the department that oversees a state’s legal cannabis or hemp program. For more 
information about what type of fungicides can be used in your state, please reach out to the governing body for your state’s 
program.
Learn More with Verne Bio
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Interested in learning more about downy mildew and other problematic things that can harm your cannabis crop? Verne Bio is the 
place to go for information regarding all things cannabis. We offer a wide range of educational content for growers of all 
experiences. When you visit the Verne Bio website, you can expect to find content covering;
● Hop Latent Viroid
● Cultivation tips and tricks
● Viruses and Viroids
● Molds that commonly impact Cannabis crops
This content and more can be found on our blog and education center located directly on our website. To stay on top of all the 
latest news from Verne Bio, readers can sign up for our newsletter located on our home page. This will bring the newest articles, 
information about sales, and more directly to your inbox.
 
With Verne Bio in your corner, problems like downy mildew don’t have to seem so daunting. Our goal is to help provide cannabis 
industry professionals with the tools they need to succeed. Just like you, cannabis is our passion. We combine that passion with 
science to help growers all over the US to tackle the modern problems facing the industry.
Works Cited
Olis, Jim, and Brian Hudelson. “Downy Mildew – Wisconsin Horticulture.” Wisconsin Horticulture, 6 August 2012, 
https://hort.extension.wisc.edu/articles/downy-mildew/.
Accessed 30 July 2023.
Westerveld, Sean. “Downy Mildew Identified on Hemp (Cannabis sativa) in Southwestern Ontario.” onspecialtycrops, 22 September 
2020, https://onspecialtycrops.ca/2020/09/22/downy-mildew-identified-on-hemp-cannabis-sativ a-in-southwestern-ontario/. 
Accessed 30 July 2023.
West Virginia University Extension. “Downy Mildew | Extension | West Virginia University.” WVU Extension, 4 August 2022,
https://extension.wvu.edu/lawn-gardening-pests/plant-disease/fruit-vegetable-diseases/d owny-mildew. Accessed 30 July 2023.
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