Top 5 Facts About Mildew on Cannabis


Cannabis is one of the oldest herbs known to humankind. Evidence of marijuana use in the human population has been dated as far back as 2,500 years ago. And while weed was initially viewed with a degree of disdain, the past few years have witnessed significant strides toward mainstreaming various extracts from the plant. 

That explains why the marijuana industry is booming too. According to Fortune Business Insights, the global weed market size was estimated at USD 28.288 billion in 2021 and is projected to hit USD 197.74 million in 2028. 

One of the best things about investing in the cannabis industry is that there are multiple ways to enter the market, such as becoming a marijuana cultivator. 

But as with any farmer, cannabis growers have numerous challenges to contend with. One such challenge is the problem of mildew. 

This post takes a closer look at mildew in cannabis by highlighting the top five facts about it.

  1. Cannabis Mildew Is a Parasitic Fungus

Like most plants, cannabis can be affected by numerous pathogens. These include bacteria, viruses, parasites, and fungi. Understanding the specific pathogenic agent that your weed plants are suffering from is key to prescribing the right remedial or mitigative measures. 

Now, cannabis mildew is a parasitic fungus. The fungus goes by many other names, including white mold, Oidium, or powdery mildew on cannabis (commonly abbreviated as PM). 

Powdery mildew fungi, including those that attack cannabis plants, are technically classified as Ascomycetes. In taxonomy, these organisms belong to the Erysiphales order.

Being an obligate parasite, cannabis mildew obtains all its nutrients from the living tissues of cannabis plants. Which means the fungus cannot thrive independently of its host. And that’s one of the things that makes it so dangerous. 

Powdery mildew is actually considered the most destructive cannabis pest. So, early interventions are paramount to salvage the situation before it gets out of hand.

  1. Various Organisms Can Cause Cannabis Mildew 

Mildew in plants can result from various organisms. In cannabis, the disease is usually caused by the fungus Golovinomyces ambrosiae

Some cases may also be attributable to the pathogen Podosphaera macularis, especially under outdoor conditions. 

However, when it comes to cannabis mildew, the focus should be on the aggravating factors and not the actual causative agents. 

Various environmental factors may exacerbate marijuana PM. The most notable one is high relative humidity. Note that relative humidity is also associated with vapor pressure deficit (VPD), which is the difference between the ambient water vapor in the air and the maximum amount of water vapor that air can hold when saturated. The lower the VPD levels, the higher the humidity and the higher the risks of bud rot.

 Other environmental factors that may cause bud rot include low (but not freezing) temperature conditions and poor or unfiltered airflow. Therefore, be sure to maintain the ideal cannabis growing conditions at all times. In the case of cannabis, typical ideal conditions would be a temperature range of approximately 68 to 77 degrees Fahrenheit, a relative humidity of about 40% to 60%, and a VPD within the range of 0.6–1.5 kPa. However, these conditions can vary significantly depending on the growth stage of the cannabis plant.

  1. Cannabis Mildew Often Remains Hidden Until Damage Occurs

Most cannabis pests and diseases are easy to treat when detected early enough. Moreover, regular inspection can prevent invasive pests from taking a toll on your cannabis farm and causing irreparable damage. 

Unfortunately, cannabis mildew has a notorious reputation for being a hidden threat. By the time this fungus begins to manifest visible symptoms, it usually has already taken a heavy toll on your marijuana plants. 

This fungus acts as an obligate biotroph, penetrating your cannabis plant tissues and often remaining undetected for several weeks.  

Cannabis mildew generally emerges and sporulates about two weeks into the flowering stage. Note that this is the most important growth phase for cannabis plants. So, you can only imagine the extent of damage the pest can cause to your mature weed buds. 

Depending on the number of plants affected, the impact could have serious economic repercussions.

Close up of Marijuana Cannabis Plant with Visible Trichomes
  1. Cannabis Mildew Resembles a Powdery Dust

Cannabis mildew may take significantly longer to produce noticeable signs. However, the symptoms are unmistakable when they finally begin to manifest themselves. 

White dusting-like flour is the tell-tale sign of mildew on cannabis. The symptom will begin as a powdery, circular spot on the leaves and stems of your weed plants. 

You may notice that the signs are concentrated on the aerial sections of the leaves. But depending on the disease’s progression, the leaf undersides may also be affected. 

Without swift remedial action, cannabis mildew will continue to spread to other sections of the crop, leaving a trail of destruction in its wake. From leaves and stems to flowers, buds, and petioles – no part of a cannabis plant is immune from this invasive fungus. 

Cannabis mildew will quickly spread to other plants on the farm if not treated. Even worse is that this highly-resilient fungus can remain dormant in its surroundings for months and resurface when the conditions are right, which is almost always when your marijuana plants are nearing harvesting.

white mold on the plant cannabis
  1. Cannabis Mildew Is Manageable 

Despite its destructive nature, it’s reassuring to know that cannabis mildew is perfectly manageable. One way to manage the disease is by applying fungicides to the infected plants. 

However, note that many states are outlawing fungicides due to their adverse effects on cannabinoids and adjacent plants. That takes us back to early detection. 

Conducting routine inspections in your cannabis garden can be instrumental in picking up the signs of mildew before the disease progresses beyond manageable levels. 

The most effective early intervention measures include removing the diseased leaves and maintaining ideal growing conditions to contain the spread of the disease.

Drying buds on a spray rose. Improper rose care, powdery mildew and aphids. Pink sawfly, close-up. Pruning dead rose buds.

Wrap Up

Powdery mildew is a leading cause of cannabis crop failure worldwide. However, early detection and treatment can prevent these parasitic fungi from wreaking havoc on your cannabis farm.