The Top 3 Cannabis Pathogens that Plague Cannabis Cultivators

They say that an ounce of prevention is worth a pound of cure. While cannabis cultivation may seem simple to the layman it is riddled with potential bumps in the road that could be extremely costly. Plant pathogens are just one of these bumps and they can lead to inferior product and can also greatly reduce yield and/or total crop loss if not dealt with swiftly. Early detection of such pathogens is paramount.  

Powdery Mildew

We’ve all seen it before – it’s that pesky white film on cannabis leaves and flowers that leaves a less than stellar bag appeal and makes for a harsher smoke with bad taste. Powdery mildew is a pain for cultivators to deal with once their grow has been afflicted and it is even harder to eradicate. Beyond being difficult to control and remove, powdery mildew is an obligate biotroph meaning that it cannot survive without taking nutrients from its host. This leads to a lackluster product with less robust trichomes resulting in lower cannabinoid and terpene production. It was initially reported by John MacPartland that the causal agent of powdery mildew was P. macularis however after numerous failed attempts by the Medicinal Genomics team to detect it using primers specific to this pathogen we decided to whole genome shotgun this pathogen. It turns out, the powdery mildew that infects cannabis is a unique species which is why the primers from the literature failed to amplify. This work revealed that its genome’s internal transcribed spacer (ITS) region sequence is 98% identical to P.macularis and Golovinomyces (the type of PM that infects grapes). This is a novel species that has been coined cannabis derived powdery mildew or CDPM which is not found on public genomic databases like NCBI therefore primers specific to this genome were developed by Medicinal Genomics.

Research performed on Arabadopsis demonstrates that some powdery mildews have a 4-7 day post inoculation (dpi) window where it remains invisible as it builds a network internal to the plant¹.  Research performed in Humulus lupulus or hops, a close cousin of cannabis has demonstrated incubation periods up to 49 days with P.macularis². 

Botryis Cinerea

If you’ve ever beheld a beautiful bud only to break it open and find fuzzies on the inside this is likely Botrytis cinerea or gray mold which arguably one of the most destructive cannabis pathogens. It is a necrotrophic fungal infection that typically affects cannabis in environments with excessive humidity. This infection typically forms on the inside of the buds and is most commonly found in cannabis grown outdoors particularly in coastal regions. Botrytis is systemic and can be passed down through seeds and often coinfects with powdery mildew³. A recent survey of the microbial survey of a commercial outdoor cannabis grow in Colorado demonstrated that this is the most common fungal organism present and accounted for up to 34% of the fungal sequence identified⁴. However, it can also manifest in grow rooms that do not have their humidity under control. 

Fusarium

Ever had a cannabis plant that seemed healthy and thriving initially and suddenly you flowered it and it just dudded out? The likely culprit could be Fusarium spp. which are commonly encountered in the cannabis microbiome however the species F. oxysporum and F. solani seem to be especially problematic for cannabis cultivators⁵. Fusarium belongs to the Nectriaceae family and encompasses a broad genus that includes different filamentous fungi that live in the soil. Generally, these types of fungi are saprotrophic meaning they feed on the waste of other organisms. They act on the plant in a parasitic manner – feeding on it and causing it to get sick and, at times, die. Fusarium attacks from the soil and colonizes the cannabis plant through its xylem conduits, blocking the flow of sap. To draw a comparison to the human body, it would be as if this fungus blocked the veins, preventing the flow of blood. It is so effective at devastating certain types of crops that at one time the federal government was investigating the use of F. oxysporum as a means of eradicating cannabis grows domestically and coca crops in Colombia^6-7. It is important to note that there are some non-fungal infections (viroids) that present similar symptoms to fusarium, notably presenting dudding symptoms and failure to thrive. 

In addition to acting as a plant pathogen, fusarium also poses health risks to humans. In particular F. oxysporum is the causal agent of fusariosis, a condition that presents radiologic similarities Aspergillosis producing alveolar infiltrates, nodules with or without halo sign, ground-glass infiltrates, and pleural effusions. This fungal pathogen is mostly a concern for those who are immunocompromised and once contracted it is often fatal ^8-13. 

How to Prevent These Costly Infestations

As mentioned at the beginning of this article, the best way to prevent crop losses from these cannabis pathogens is to avoid contaminating your grow environment in the first place. Some precautionary measures that can be taken include filtration of all incoming air from the outside environment to prevent spores from entering your grow. Secondly, decontaminating yourself prior to entering your grow by utilizing clean room protocols and proper preventative gear (gowns, booties, etc.) can greatly reduce your chances of afflicting your operation with these pathogens. Additionally, implementing a testing regimen to continually monitor your cultivation site helps to ensure that the infected crops are quickly identified and swiftly quarantined. This can be done by both qualitative and quantitative fashion with either traditional PCR or quantitative PCR methods.

References

1. Webling R, Panstruga R. Rapid quantification of plant-powdery mildew interactions by qPCR and conidiospore counts. Plant Methods. 2012;8(1):35.
2. Twomey MC, Wolfenbarger SN, Woods JL, Gent DH. Development of partial ontogenic resistance to powdery mildew in hop cones and its management implications. PLoS ONE. 2015;10(3):e0120987.
3. Shaw MW, Emmanuel CJ, Emilda D, et al. Analysis of Cryptic, Systemic Botrytis Infections in Symptomless Hosts. Front Plant Sci. 2016;7:625.
4. Green BJ, Couch JR, Lemons AR, et al. Microbial hazards during harvesting and processing at an outdoor United States cannabis farm. J Occup Environ Hyg. 2018;15(5):430-440.
5. Punja, Zamir & Rodriguez, Gina. (2018). Fusarium and Pythium species infecting roots of hydroponically grown marijuana ( Cannabis sativa L.) plants. Canadian Journal of Plant Pathology. 40. 10.1080/07060661.2018.1535466.
6. K. S. Tiourebaev  GVS, M. Dolgovskaya , M. K. McCarthy , T. W. Anderson , L. D. Carsten , A. L. Pilgeram  & D. C. Sands. Biological Control of Infestations of Ditchweed ( Cannabis sativa ) with Fusarium oxysporum f.sp. Cannabis in Kazakhstan. Biocontrol Science and Technology 2011;11(4).
7. Robbins, Jim (2000, July 18) “Drug War Awaits Attack of Killer Fungus” https://www.nytimes.com/2000/07/18/science/drug-war-awaits-attack-of-killer-fungus.htm
8. Khwaja S, Yacoub A, Cheema A, Rihana N, Russo R, Velez AP, et al. Marijuana Smoking in Patients With Leukemia. Cancer control : journal of the Moffitt Cancer Center. 2016 Jul;23(3):278-83. PubMed PMID: 27556668.
9. Nucci M, Anaissie E. Fusarium infections in immunocompromised patients. Clinical microbiology reviews. 2007 Oct;20(4):695-704. PubMed PMID: 17934079. Pubmed Central PMCID: 2176050.
10. Nucci M, Marr KA, Queiroz-Telles F, Martins CA, Trabasso P, Costa S, et al. Fusarium infection in hematopoietic stem cell transplant recipients. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 2004 May 1;38(9):1237-42. PubMed PMID: 15127334.
11. Nucci M. Emerging moulds: Fusarium, Scedosporium and Zygomycetes in transplant recipients. Current opinion in infectious diseases. 2003 Dec;16(6):607-12. PubMed PMID: 14624113.
12. Nucci M, Anaissie EJ, Queiroz-Telles F, Martins CA, Trabasso P, Solza C, et al. Outcome predictors of 84 patients with hematologic malignancies and Fusarium infection. Cancer. 2003 Jul 15;98(2):315-9. PubMed PMID: 12872351.
13. Nucci M, Anaissie E. Cutaneous infection by Fusarium species in healthy and immunocompromised hosts: implications for diagnosis and management. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 2002 Oct 15;35(8):909-20. PubMed PMID: 12355377.