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Decontaminating Medical Cannabis in Israel: How to Remove Fungal Pathogens

by Shachar Jerushalmi (PhD student), Marcel Maymon (research assistant), and Stanley Freeman (PhD, senior researcher)
Published: Apr 09, 2020   

Image credit: the Agricultural Research Organization of Israel

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The use and research into medical cannabis is becoming more common. However, there is a lack of knowledge regarding fungi that infect cannabis plants, and few practical and effective methods for managing the casual agents of disease.

Since medical cannabis is used by patients with weakened immune systems, there are potential risks to their health when exposed to microbial-infected cannabis1 (fungal spores, bacteria, etc.). Therefore, it’s of crucial importance to supply medical cannabis-treated patients with a clean, disease-free, and healthy product.

Fortunately, in Israel, the authorities of the Israel Medical Cannabis Agency and the Ministry of Health have been working to establish regulations that will set guidelines and standards for medical cannabis growers, including disease management protocols.

In this study, researchers from Israel’s Agricultural Research Organization measured how three different sterilization methods (gamma irradiation, beta irradiation, and cold plasma) reduced fungal colony forming units (CFUs) in naturally infected commercial medical cannabis inflorescences.

All the tested methods significantly reduced CFUs to those below regulated threshold levels permitted by the Israeli Medical Cannabis Agency of the Ministry of Health.  

The challenge

There are still many challenges regarding plant diseases when cultivating medical cannabis:

  1. lack of formal and professional knowledge of fungi that infect medical cannabis plants and practical and efficient methods for dealing with them
  2. strict limitations for the use of pesticides in the fear of exposing patients consuming medical cannabis to chemical residues
  3. regulations regarding the maximum levels of total yeast and mold (TYM) colony forming units (CFUs) permitted in medical cannabis inflorescences, crucial for the quality of medical cannabis inflorescences dispensed to patients.

Although sterilization using autoclaving or UV irradiation may commonly be used, the most important therapeutic compounds in medical cannabis (cannabinoids and terpenes) are heat and light sensitive and undergo decarboxylation, causing early decay when exposed to the above decontamination conditions. This emphasizes the necessity for novel ways to disinfect medical cannabis without exposing the product to high temperatures or UV irradiation.

Based upon reports from medical cannabis cultivation farms in Israel, extensive damage to medical cannabis yield is due to various plant pathogens, the management of which remains unanswered by Ministry of Agriculture. In light of this situation, the main objectives of our study were to:

  1. Identify and characterize the major medical cannabis fungal pathogens in commercial farms in Israel.
  2. Examine novel ways to reduce infection levels in inflorescences using different disinfection methods: i.e. gamma and e-beam irradiation (ionizing radiation that leaves no residues and does not involve extreme heat or UV irradiation after application) and cold plasma (ionized gas with a limited net charge and addition of H2O2 for the disinfection mechanism) of dried and consumer-ready commercial products.

E-beam (beta irradiation) and gamma irradiation were conducted at Sorvan Radiation Ltd., Soreq Nuclear Research Center, Israel. Cold plasma treatment was conducted using a prototype created by NovaGreen company.

The results

Two major foliage pathogens were identified, Alternaria alternata and Botrytis cinerea in leaves and inflorescences (Figures 1). Two major stem and soilborne pathogens were identified, Fusarium oxysporum and F. solani. Other important mycotoxin-producing fungi that can directly harm patients included aspergillus, fusarium, penicillium species, and Trichothecium roseum, that were isolated from symptomatic foliage.

Figure 1. Inflorescence mold caused by the fungus Alternaria alternata.

Gamma irradiation reduced TYM CFU levels by approximately 6- and 4.5-log fold, e-beam treatments reduced TYM CFU levels by approximately 5-log-fold while a 10 min exposure to cold plasma treatment resulted in 5-log-fold reduction in TYM CFU levels, in both naturally infected and artificially inoculated B. cinerea medical cannabis inflorescences2 (Figure 2).

Figure 2. CFU reduction in naturally infected inflorescences using different dosages of E-beam irradiation.

Discussion and conclusions

In Israel, Alternaria alternata was found to be the major pathogen affecting inflorescence in general, followed by Botrytis cinerea and Fusarium oxysporum. Our results are interesting compared to a similar research conducted in Canada, in which the most common medical cannabis inflorescence pathogens found were Botrytis cinerea, Fusarium solani, Penicillium olsonii, F. oxysporum, F. equiseti and Penicillium copticola, whereas Alternaria alternata was not detected3. This could be explained in part by the differences in location, environment and climate, highlighting the importance of local research and data collection.

In order to achieve a high level of quality control, many countries, including Israel, the Netherlands, and the European pharmacopoeia have imposed strict regulations restricting the permitted number of microbial contaminations present in commercial medical cannabis to, 2000, 100 and 50,000 colony forming units CFUs of total yeasts and molds per gram of inflorescence, respectively4.

Although gamma irradiation was very effective in reducing TYM CFU levels to below those levels specified, it’s the most expensive and complicated method for medical cannabis sterilization. However, both e-beam and cold plasma treatments have greater potential since they are cheaper and simpler to apply, and are equally effective for medical cannabis sterilization.

Thus, both of these methods appear to possess the potential in producing clean, safe, and healthy medical cannabis products. Future studies will focus on the effect of these disinfection methods on the active compounds in medical cannabis inflorescences, to assure that they have no adverse effects on the commercial product administered to patients. In addition, assessing fungicide and mycotoxin degradation effects by cold plasma and e-beam in medical cannabis inflorescences in vivo, will require further, extensive research.


  1. Gargani, Y., Bishop, P., and Denning, D.W. (2011). Too many mouldy joints - marijuana and chronic pulmonary aspergillosis. Mediterranean Journal of Hematology and Infectious Diseases 3, e2011005.
  2. Jerushalmi, S., Maymon, M., Dombrovsky, A., and Freeman, S. (2020). Effects of cold plasma, gamma and e-beam irradiations on reduction of fungal colony forming unit levels in medical cannabis inflorescences. Journal of Cannabis Research, 6, 0–11. doi: 10.1186/s42238-020-00020-6.
  3. Punja, Z.K., Collyer, D., Scott, C., Lung, S., Holmes, J., and Sutton, D. (2019).  Pathogens and molds affecting production and quality of Cannabis sativa L. Frontiers in Plant Science 10, 1120. doi:10.3389/fpls.2019.01120. 
  4. Hazekamp, A. (2016). Evaluating the effects of gamma-irradiation for decontamination of medicinal cannabis. Frontiers in Pharmacoogy 7, 108.


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