The Facts: Mold and Mildew in Cannabis Cultivation and How to Prevent it

The cultivation of cannabis is naturally a hot and humid hobby, lending itself to the germination of mold, mildew, and fungus. White powdery mildew and bud rot are not uncommon sights for growers, especially newbies. If you've been growing cannabis at all, you may have encountered one or more of these crop-destroying infections.

Growing indoors creates a perfect environment for microbials (mold, mildews, etc.) to flourish. What does this environment look like?

Cannabis grows best with exposure to temperatures around 75°F (25°C), and so too do molds. Combined with botanical transpiration (the movement of water from roots, through leaves, and into the air), the indoor growing environment is hot and damp during the day.

Stopping the spread of microbials requires a clean grow room, a sound HVAC system, and in the worst-case scenario, early detection. These are the three basic principles, when mastered, will reduce the risk of lost time, money, and harvest.

What's the Risk?

The risk of mold and mildew infection to your crop exists on a spectrum. At the lowest end, you may contain the spread of disease by removing one or two plants. Imagine the time and money already invested, plus future loss of the harvested bud. 

The more considerable risk is if the infection spreads throughout a room or an entire facility. Mold and mildew can decimate a whole harvest and have massive financial repercussions.

Reducing the spread of contamination becomes increasingly difficult the larger the initial infection. Decontamination efforts are labor-intensive, time-consuming, and costly. One mold issue can have months-long repercussions for a commercial grower.

But the risk doesn't stop with the grower. If the contaminated product reaches the final consumer, whether that be a friend or a paying customer, there are health risks. We've all read the headlines coming out of legal markets like Nevada, where dozens of strains failed mold and yeast testing. Dozens of products required recall, as they had already reached the hands of customers. 

Mold and mildew, when inhaled, are dangerous. Ongoing exposure can cause severe respiratory infection, tissue damage, and even death. 

What is White Powdery Mildew?

White powdery mildew is technically a fungus. It is an obligate parasite. There are several different types, each favoring certain species over others. As an obligate parasite, it needs living plant tissue to survive, and once the host plant dies, it too will eventually die out (unless new material presents itself).

Experts at Medicinal Genomics have sequenced several strains from across North American to discover that the powdery mildew, which infects cannabis, is genetically different from types that target other plants. There may be several different strains that live on cannabis in North America.

As a fungus, it grows a mycelium network on and within its host. Mycelium is the microscopic network fungal cells making up the organism.

Medicinal Genomics suspects that even if no white powdery mildew is visible, there is a high chance a host plant may carry the fungal infection within its stem, leaves, and root systems. Normally, these remain benign or dormant, waiting for the right conditions to launch their attack.

This fungus thrives with relatively high humidity and in temperatures between 70 to 80°F (22 to 27°C). If you are paying attention, this is the same ideal environment for growing cannabis.

When the climate becomes hospitable, the white powdery mildew organism kicks into action to release spores. Spores are the reproductive phase and one of the first signs of visible infection.

The signs of powdery mildew include:

  • Contorted and misformed leaves
  • Wilted or dying leaves
  • White powdery spots on leaves, flowers, or other parts of plants

What is Bud Rot?

A second widespread problem facing cannabis cultivators is bud rot, also known as grey mold or Botrytis. It is another fungal infection, which again can infect many different crops, well beyond cannabis.

The form which attacks cannabis is called Botrytis cinerea, which roughly translates to grape disease in Ancient Greek. Unsurprisingly, Botrytis is a common infection in vineyards.

Although bud rot may appear in both indoor and outdoor facilities, it is more common in outdoor operations. Indoor farmers should have better control over climate while outdoor farmers may be at the whim of the rain and humidity. For example, if it rains, followed by a long humid period, plants remain damp, creating the perfect microclimate for fungal growth.

Although this fungus can technically grow in temperatures ranging from 28°F to 90°F degrees, it explodes between 70° to 77°F. Boosted by high humidity levels, it can go from zero to sixty in a matter of days.

The signs of bud rot include:

  • Small brown spots on leaves surrounding flowers
  • Fluffy, spider-web like growth on flowers
  • Browning, greying, or darkening of flowers
  • Signs of drying out (browning, outside is brittle)
  • Rotting, mushy interior of the flower

What Other Molds Contaminate Cannabis?

As the legal market grows, testing improves, and so too does our scientific understanding of mold, mildew, and cannabis.

According to a piece by Ed Rosethenal, cannabis-cultivator extraordinaire, laboratory testing has uncovered a complicated cocktail of bacteria and molds on cannabis plants. Some of the most common are:

  • Klebsiella
  • Enterobacteria (E.coli)
  • Acinetobacter 
  • Aspergillus 
  • Fusarium
  • Mucor
  • Penicillium
  • Botryotini

After testing 17 distinct cannabis samples, a recent study detailed each contained a complex environment filled with several mycotoxins. Some of the most common, as per the assessment, were: Aspergillus versicolor, Aspergillus terreus Penicillium citrinum, and Penicillium paxilli.

On the surface, these findings seem terrifying about the potential for contamination, but we have to remember that fungal spores are everywhere. Fruiting fungal bodies emit millions of spores per day. Spores are microscopic in size and travel over long distances.

According to "One potentially conservative estimate," humans breathe "between 60–60,000 spores per day; the higher number is relevant to moldy buildings or the outdoors." So finding some fungal contamination on these samples should be expected.

But, spores become dangerous if given a hot and humid environment. Combine fungal spores with ideal climate conditions, and there is potential for dangerous growth. 

Throughout the entire process, from cultivation to curing, growers must stay on top of humidity, temperature, and environment to keep out mold and mildew.

How to Combat Mold in the Grow Room

There are three protocols for beating mold in the grow room:

  • Cleanliness in the Grow Room
  • Climate Control and HVAC
  • Early detection and Elimination

Cleanliness in the Grow Room

A clean environment starts with the sourcing of quality clones and soil (or learning to mix your own). Disinfect all equipment entering the space and implement routine cleaning of all equipment.

As Cannabis Industry Journal recommends, isolate the dirtiest of tasks and compartmentalize the grow room as much as possible. The fewer chances spores have to travel, the less risk they pose to the plants. The adage "Cleanliness is next to godliness" definitely applies here.

Climate Control and HVAC

Because mold and mildew love a hot, wet climate, regulate the growing environment to the cooler, drier end of the spectrum. If there is standing water sitting around the grow room, immediately dispose of it, as it will evaporate to increase humidity levels. Ed Rosthenal and others have suggested keeping the humidity below 50 percent. 

The second aspect of climate control is through an HVAC system, which manages the atmosphere. Tinkering with the controls and settings will impact the plant's transpiration rate, regulating humidity levels to some extent.

All HVAC have an air filtration system, which requires regular cleaning and maintenance. Some systems will come with an additional mold filtration system, which can help further reduce the risk. 

How to monitor and manage the climate and HVAC? Install a Pulse One Environmental Control, which controls temperature, vapor pressure deficit (VPD), and relative humidity, but also integrates with the lighting system and has a journaling option. All for total remote control of the growing environment.

Early Detection and Elimination

A third and final way to combat mold and mildew in the grow room is constant daily monitoring. While an outbreak of fungal spores can happen overnight, very likely, you'll notice the telltale signs well before the organism releases spores. 

Check plants daily for signs of disease. Get into the deep canopy, inspect the buds, and undergrowth. Look for discoloration, rot, and spots. As the flowers develop, use an LED Loupe to get close up and personal with the bud. 

If you notice any signs of infestation, immediately remove the plant from the room to prevent the issue from spreading. Sacrificing one plant is worth it to save the full harvest.

At this time, it may also be worth a chemical intervention to prevent the spread among the remaining plants. Explore cannabis-specific treatment options, as these will be best suited to keeping the bud safe for post-harvest consumption.

How to Avoid Mold During Drying Curing

Many of the molds and mildews which fall into the 'other' category will only become a concern post-harvest. With improper drying, curing, and storage practices, these pathogens can turn a good bud, bad.

Again, many of these spores exist naturally but in levels that aren't dangerous. However, if they accidentally find themselves in a warm and moist environment, all bets are off. 

During drying, Rosenthal recommends a fast drying rate during the first three days. To achieve this, he suggests starting at 68° F (20° C) with 55 percent relative humidity. In the second phase, drop the temperature and relative humidity slightly to slow down the drying. Always keep air flowing throughout the harvest to avoid damp patches.

The curing process is another phase and yet another opportunity for lingering spores to explode into action. According to Rosenthal, "Curing is an art and should be tried with small batches first."

Use airtight glass containers and maintain the humidity between 45 to 50 percent. In the early days, burp frequently to express the natural gasses. Pay attention to the aroma of the curing jars during this stage, as it will be a key indicator if a batch begins to sour.

Mold and Mildew Resistant Strains

There is an entire industry dedicated to developing strains of cannabis resistant to mold and mildew. Two factors play into mold resistance: regionally specific breeding and genetics. 

Local breeders and suppliers grow within the same environment you do, meaning their genetics are accustomed to the temperature, humidity, and weather conditions of your region.

Buying local for outdoor cultivation is immensely valuable for mold and mildew resistance. For example, a clone supplier in the Nevada desert might have a much different selection than someone growing in the rainy Pacific Northwest.

Genetics is the second factor. Genetic experts in the industry, like Medicinal Genomics, are making this work easier through DNA sequencing. Naturally, certain plants will demonstrate more resistance to fungus than others.

Breeders select for these traits improve upon the strain's overall genetic resistance to mold and mildew. Over time, certain chemovars will become better at fighting off fungal infection, even in damp conditions.

Fighting Mold and Mildew with Smart Cultivation Practices

Mold and mildew are common occurrences in cannabis cultivation, but growers can mitigate the risks with the right practices. First, know the facts about white powdery mildew, bud rot, and the risk of post-harvest infection. What temperatures and climates tip the scale to favor a fungal explosion?

Second, practice clean and smart cultivation. If you set your space up for success from the beginning, mold and mildew will struggle to take hold. Use technology, like the Pulse One Environmental Control, to stay on top of your environment. Get an  LED Loupe to make early detection easy.

Finally, take action as soon as you suspect the development of mold. Waiting even a day to see what happens can mean total devastation.





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