This is the third article in series of articles on the differences between outdoor versus indoor grown cannabis. In this article, we discuss mold in general followed by a brief look at fungicides (sprays that kill mold). We finish by discussing why outdated petri dish testing for toxic microbes is best replaced with cutting-edge DNA testing.


Bud Rot
Mold spores are everywhere. When the weather is warm and wet, mold spores sprout into fungal blooms. Together with bacteria, mold is natures way of getting rid of the dead and dying. Without the two breaking down organic matter, there would be heaps of dead trees, plants, insects, animals, and the like lying around everywhere. The two are the great recyclers. Unfortunately, mold is always on the lookout for an opportunity to feast, and this sometimes includes living plants.

We had record-breaking wet weather in Wisconsin in 2019. As a consequence, the risk of fungal outbreaks like powdery mildew loomed continuously in the background. The plants did well without any intervention except near the end of the season when we did see some instances of isolated bud rot. Given that mold is forever looking for the slightest inroad, the buds of some plants weren’t strong enough to resist in that exceptionally wet weather.

Happily, the bud rot stayed isolated. In part this was because we had selected strains that were resistant to fungi. In addition, we sprayed and drenched the soil with organic compost teas along with having fed nutrient rich organic fertilizer. This care not only made the plants stronger but enriched the soil. All these factors along with naturally competing microbes that exist outdoors helped to limit the extent of the rot.

By diligently and regularly removing the offending buds, the plants stayed healthy and the loses were slight. This was all done without any fungicides. Well, we did apply one application of OMRI certified organic sulfur when the plants were small and before the wet weather. This was done mostly to ensure we had a viable approach to dealing with a serious mold outbreak should it occur. It’s worth noting that besides being a preventative for mold on plants, sulfur has been used to treat a variety of health conditions in people for centuries. We take great care in selecting what goes on our plants.

Forgoing spraying fungicides would not have been an option in an indoor operation where there aren’t any competing microbes to help keep mold in check. This is combined with the facts that indoor grows are kept warm (70-90°F), humid (50-90% RH), and with CO2 levels between 2-4 times normal outsides levels (700-1,500 ppm). All these factors tend to exacerbates fungal growth and often necessitates the increased use of fungicides indoors. Our understanding is that given the increased risk of fungal outbreaks indoors, many indoor grows spray fungicides routinely.

Now this isn’t to say that some outdoor growers don’t routinely spray fungicides too. However, it appears that mold is generally more of a problem for indoor grows. In Health Hazards of Indoor Pot Grows, they found that 20 out of 30 indoor grow operations had extreme levels of mold and spores to the extent that “petri dishes and field-testing equipment ‘topped out’ and couldn’t record the unexpectedly high levels”. As an aside, it’s worth noting that the overuse of fungicides is likely behind the development of deadly super molds like Candida Auris that are inundating our hospitals and even killing patients.

Should cannabis become moldy, it probably goes without saying that using that product isn’t healthy for anyone, especially those with compromised immune systems. There are documented cases of of unhealthy individuals being killed by smoking moldy cannabis. Neither mold nor many of the sprays used to fight it are healthful.

At a minimum, our recommendation is to make sure to purchase cannabis from a grower that discloses what fungicides were used and that only uses OMRI approved certified organic sprays. At Organic Entourage we do all of this along with providing third-party mold lab results. Not only can you see for yourself how we grow our CBD hemp, but we provide lab results to back up our works.

Microbial Plate Testing

Microbial testing for bacteria and fungi/mold has been around for a long time. In general, microbial testing on foods is done by placing a diluted amount of the product being tested into a petri dish where it is allowed to incubate for a specific amount of time. The petri dishes (plates) have a food source know as “agar” in them and upon which the diluted substance is place. If there are mold spores in the dilution being tested, those spores will bloom.

In the case of mold and yeast testing, the agar is chosen to inhibit bacterial growth while promoting mold spore sprouting and yeast budding. That way supposedly what’s growing on the dish is mostly mold. At the end of the incubation period, the numbers of the various types of microbes are counted by a technician. This method of plate counting is one of two ways cannabis is tested for bacteria and mold/yeast.

Plate counts have been around for over 100 year and Standard Plate Counts (SPC) methodology, that uses specific agars, has been in place since 1960. In terms of cannabis, states like Colorado, Nevada, Maine, Illinois and Massachusetts along with Canada require cannabis to be tested via two different plate counts. The first is a Total Aerobic Microbial Count (TAMC) and the other is a Total Yeast and Mold Count (TYMC). The TAMC tests for the overall concentration of bacteria. The TYMC tests for the overall concentration of yeasts and molds.

While this may sound all well and good, there are some serious limitations to plate counts. The article 5 Reasons Why Culture Plating Should Not be Used for Microbial Safety Testing on Cannabis does a nice job covering these issues. The relevant points are summarized below.

Plate Testing Limitations

  1. Bacteria Grow on Mold Plates Skews Results – in spite of using specialized agars, up to 60% of what grows on the TYMC plates is bacteria – not yeasts and molds. TYMC stands for testing of Total Yeast and Mold Counts. Since the technician simple counts up the total number of “colony forming units”, the mold and yeast counts will include bacterial colonies and consequently, will be skewed high. An analysis of plate cultured cannabis found substantial growth of Clostridium botulinum and other bacteria were frequently observed on one or both of the culture-based Total Yeast and Mold (TYM) platforms. The presence of plant growth promoting (beneficial) fungal species further influenced the differential growth of species in the microbiome of each sample.
  2. Plate Counts Miss Harmful Molds – plants develop a natural relationship with a large range of bacteria and fungi that actually live within the cells of the plant as opposed to growing on the surface of the plant. These types of microbes are called endophytes. Many endophytes are beneficial to plants. Some endophytes are harmful to people.For example, the endophyte fungi Aspergillus Niger is harmful to human health. When it comes to plate counts, unfortunately they don’t come anywhere close to breaking down the plant cell walls (lyse). As such, plate counting does not count endophytes like the harmful Aspergillus Niger while including many fungi and yeasts that are beneficial to people.
  3. Non-Representative Growth – in spite of trying to pick a food source that feeds a wide variety of the microbes being tested for, only a fraction of bacteria or fungi can actual grow on the agar and even fewer thrive. This limitation is further exacerbated by the fact that bacteria and fungi fight with one another all the time. They do this by exuding chemicals that makes the environment less habitable to their competitors. The chemicals they emit kills off other bacteria and fungi that would otherwise eat their “yummy” food source. An analysis of plate cultured cannabis found samples subjected to culture showed substantial shifts in the number and diversity of species present, including the failure of Aspergillus species to grow well on either platform.. As such, simply counting the overall number of microbes present can be rather misleading. What if there is a really toxic fungus stymied by competitors and therefore not properly represented on the plate count? What if a benign bacteria floods the plate?

The issues described with plate counts significantly diminishes their value. To give you a sense for the very real limitations of plate testing, take a look at the Golden Tarp Award. This is a contest where top cannabis growers compete for awards. The cannabis is tested extensively including a Total Yeast and Mold Count (TYMC) and Total Yeast and Mold Count (TYMC). What experts at the Golden Tarp Award found was that out of 156 entries, 108 of them failed the microbial plate testing! (see the video Getting Out in Front of Testing at 40:00)

Judges Panel
Note: Sadly, this instructive video has been made private.

Does this mean these expert growers don’t know what they’re doing? Not at all. It simply means they are using beneficial microbes to grow robust plants. It is these beneficial and harmless microbes that are being misidentified as problematic by plate counts.

At 11:00 in the video, panel members discuss the fact that cannabis grown outdoors using biodynamic/organic practices, almost always tests too high when a simple plate count of total microbes is used. For example, at Organic Entourage, we use compost tea that is sprayed onto the leaves of the plants to both feed nutrients and promote beneficial colonies of bacteria and fungi. It’s these beneficial bacteria and fungi that keep harmful microbes away. One of the spokespersons at the Golden Tarp Award stated that, “one of the only ways (to grow without pesticides) is to grow biodynamically” – through the application of bacteria and fungi that are safe for humans while keeping harmful microbes in check.

In Microbial Counts and The Golden Tarp, the contest takes the limitations in plate counting into consideration by allowing much higher microbial counts. Specifically, they allow 10,000 CFU for total bacteria and 100,000 CFU for total mold and yeast. These higher plate counts are permitted so long as adjunct DNA testing for specific microbes harmful to humans comes back negative. Note: In Microbial Limits Comparison, the American Herbal Products Association (AHPA) allows up to 100,000 CFU for total bacteria and 10,000,000 CFU for total mold/yeast for dried unprocessed herbs. Suffice it to say that plate counting has some serious limitations.

DNA qPCR Testing

The issues with plate counting are bad enough that California has gotten completely away from plate counts. Instead, California requires that cannabis be tested for specific pathogenic bacteria and fungi using qPCR testing. qPCR assays look at DNA sequences and consequently, do not have the mis-identification and skewed errors common in plate counts. qPCR testing is very specific.

In terms of cannabis, qPCR testing is required by California for two harmful bacteria (E. coli and Salmonella) along with several toxic mold species (Ochratoxin, Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger and Aspergillus terreus). These nasty microbes are typically found on plant matter that is harmful to humans. By specifically looking for these toxic microbes, qPCR testing better represents the true safety and quality of the cannabis. In other words, when higher levels of the DNA of microbes that are commonly found on moldy cannabis is found, one may rightly conclude that product is contaminated with a whole host of fungi, bacteria, actinomycetes and mycobacteria as well as inflammagens such as endotoxins, beta glucans, hemolysins, proteinases, mannans and possibly spirocyclic drimanes; as well as volatile organic compounds. It’s like finding a “smoking gun”.

Outdoor Versus Indoor Cannabis Series

  1. Outdoor vs Indoor Cannabis
  2. Chemical vs Organic Fertilizer
  3. Cannabis and Mold
  4. Heavy Metals in Cannabis
  5. Pesticides in Cannabis

The dangers of microbial contaminants in cannabis and how to prevent mold and mildew

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