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Can pesticides fail PCR tests for black mould and microbes?


Labs and regulators want to test cannabis for more and more contaminants to ensure a high standard in the market. These tests, however, despite their critical importance, are never perfect. A difficult battle exists while (trying to) prevent a wave of aspergillosis without killing quality produce. Overcoming this issue with more detailed lab results means certain pesticides might fail PCR tests for black mould due to hidden GMOs. But that depends on each specific formulation.

Some authorized fungicides use a benign fungus as an active ingredient. These beneficials ward off invasive types of mould and mildew, maintaining yield and quality. However, some ingredients might carry contaminants or recombinant genetics, which would force a producer to remediate under strict guidelines; it is a sacrifice regardless.

Is PCR better than total colony in the face of transgenic IP?

A total CFU (colony forming unit) count is a test to measure the quantity of microbial contamination. Not without limitations, though, since we need to develop specific tests to identify the few species that can colonize. Samples are cultured on different growth media, and either human or computer counts the total colony of cells. Various colonies of mould are better identified by a PCR test, though.

At least, one would assume a molecular test superior to a CFU count. PCR tests pick up a piece of genetic material, replicate that piece until enough samples can be collected and identified. And yet, molecular tests are plagued with their own limitations. PCR, for example, cannot identify between living and dead cells without prior separation, so a benign fungus will still give a positive result.

PCR (polymerase chain reaction) test. Photo by Analogicus courtesy of Pixabay.

Modified black mould genetics, a purpose and problem

Trichoderma is a naturally occurring mould crucial for healthy agricultural soil. But the strain, KRL-AG2 by Bioworks, was at one time genetically engineered with the DNA of a black mould, aspergillus niger. Here, a limitation forms; that is, a PCR test might detect the genetics of the mould or bacteria intentionally inserted into an authorized ingredient. That particular gene can therefore lead to a false-positive result. Avoiding all of this, KRL-AG2 has a colony-forming test based on specific growth media and morphology that would not be affected by recombinant genetics.

And then, the intellectual property for transgenic Trichoderma was abandoned by Bioworks in 2009. Yet, in 2002, they listed the harmful strain of black mould as a possible contaminant. Since then, the pesticide producer has upgraded their formulation according to an email response from their Technical Services Manager.

Our current Trichoderma is not transgenic and does not include genes intentionally introduced from flavalus or aspergillus. To our knowledge, our formulation does not [and should not] contain any F. flavalus or Aspergillus niger

Erfan Vafaie, Ph.D. – Bioworks

Pesticides can be contaminated with microbes but they can also contain transgenic IP and might fail PCR tests, accordingly.

Checking each other’s work for cleaner pesticides and probiotics

Dr. Vafaie did mention new quantifiable information that suggests otherwise would be valuable to explore. Moreover, PCR tests might still give a false-positive or fail — if another company formulates their pesticides with similar recombinant technology. But pesticides, it seems, only account for part of the battle of ensuring contaminated beneficials and their genetics do not enter the farm.

Kyle Boyar, a staff research associate at the University of California San Diego Skaggs School of Pharmacy who has extensive experience in the cannabis testing field, recently published a chapter on the nuances of microbial testing in the book, Recent Advances in the Science of Cannabis. Boyar’s concerns center around mould and mildew in the cannabis field and their surprising sources.

Anecdotal reports suggest probiotic products used by certain growers have contributed to some aspergillus contamination. A lesson for cultivators, considering the contamination was allegedly only discovered through third-party tests. Albeit, authorized pesticides are controlled with more stringent regulations than probiotics and nutrients, mistakes are still possible. A peer-review process, where Licensed Producers test third-party pesticides themselves, appears to be ideal for the viability of grows and the safety of consumers.

Let us know in the comments what you think of pesticide use in the cannabis industry. Have you ever grown weed and been plagued with a case of powdery mildew?


  1. US20090104165
  2. Tut, G.; Magan, N.; Brain, P.; Xu, X. Critical Evaluation of Two Commercial Biocontrol Agents for Their Efficacy against B. cinerea under In Vitro and In Vivo Conditions in Relation to Different Abiotic Factors. Agronomy 2021, 11, 1868.
  3. Punja, Z. K., Collyer, D., Scott, C., Lung, S., Holmes, J., & Sutton, D. (2019). Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in plant science10, 1120.
  4. PMRA. RootShield Biological Fungicide, Trichoderma harzianum Rifai strain KRL-AG2. 2009. Gov Can.



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