WHAT ARE PLANT PATHOGENS?
Plant pathogens are microorganisms, including bacteria, fungi, viruses and viroids, that invade the plant, causing negativel impact growth, yield, and overall health. These pathogens can spread through contaminated soil, water, unclean work tools, or infected plant material. Once inside the plant, they disrupt cellular functions and the plant’s metabolism, which lead to symptoms like leaf discoloration, stunted growth, reduced vigor, and even crop loss. Managing plant pathogens is crucial for maintaining healthy cannabis crops and maximizing yield and quality.
Some of the most relevant pathogens, cannabis operators should be aware of include:
Hop latent viroid (HLVd)
The Viroid is a single stranded circular RNA pathogen from the Pospiviroidae familiy, which was first identified in hops in Germany in 1988 ​(Puchta et al., 1988) and has infiltrated the Cannabis industry over the last years.
The symptomes have general and specific characteristics resulting in overall reduced vigor and the typical breakage of stems associated with HLVd in its last stages.
The resulting impact on yield, cannabinoid and secondary metabolite content are detrimental, reducing the respective contents by up to 80%.
The transmission are both vertical and horizontal so the spread takes place both in the same generation and across generations. The most prevalent pathways are asexual/sexual propagation (Atallah et al., 2023), mechanic & vector transmission , which resulted in industry wide infection rates surging over 90% (Adkar-Purushothama et al., 2023).
Lettuce chlorosis virus (LCV)
LCV is a linear, single-stranded RNA virus and belongs to the Closteroviridae family and was first identified by (Duffus et al. 1996) in Lettuce and the zoonotic transition onto Cannabis was detected by (Hadad et al., 2019).
As already described by its taxonomy, the main symptomes are chlorosis of the leaves and subsequent reduction in photosynthetic capacity leading to overall reduced vigor/stunted growth.
The economic impact is not yet fully established, but first trials suggest a wide spread in the industry due to the reliance on asexual propagation in commercial cannabis.
This is also the main transmission pathway as it is transmitted via pollen / ovaries. But as with most viruses, horizontal transmission, so the intra generational spread via vectors predominantly white flies, is also a large contributor.
Cannabis cryptic virus (CCV)
CCV, a double-stranded RNA virus within the Partitiviridae family, was first identified in German hemp production by ​(Ziegler et al., 2012).
The symptomes are also characterised by general and specific symptomes. The specific ones were previously associated with the hemp-streak virus, which was not able to be identified and thus making CCV the causal pathogen for this disease. They induce leaf chlorosis in streaked patterns across the leaf and reduced vigor, again, resulting in reduced final product quantity and quality. Though the symptomes are only induced by viral presence, it also needs to be triggered by abiotic stress factors otherwise it is mostly latent​ (Righetti et al., 2018)​.
Transmission is also horizontal and vertical, but the vertical transmission is predominantly characterised by asexual propagation as seed transmission only affects 20-30% of the total offspring. Infected maternal material showed the highest transmission rates (Righetti et al., 2018).
Fusarium oxysporum & solani
These Fusarium colonies are both found in commercial cannabis production and belong to the Fusarium genus. The oxysporum variant is the dominant species is F. oxysporum, making up over 90% of analysed samples (Zamir K. Punja & Gina Rodriguez; 2018).
The first described in the commercial cannabis industry by
Punja, Z. K. (2018) in Canada and has been widely spread over the whole industy.
Inoculated plants showed visible symptomes with significant reduction in root quantity and stunted growth ultimatly resulting in wilting and dying plants for both species. F. solani showed more detrimental effects, expressing more severe root rot. Next to root rot, dry curled leafs and leaf absiscion are visible symptomes caused by the lack of water / mineral supply as the root system is highly impaired (Zamir K. Punja & Gina Rodriguez; 2018) ultimatly resulting in stem / leaf necrosis.
Transmission is mostly horizontal and only vertical if the propagation material has surface level contamination and/or ingrown conodia such as infected cuttings or seed hull contamination. The fungus also spread in the plant via the vascular tissue, which can be observed by browing of the xylem (Zamir K. Punja, 2021)
Phytium
All species of Pythium were detected in commercial production especially in propagation settings
(Zamir K. Punja & Gina Rodriguez; 2018)(McPartland,1996). They are not real fungi but Oomycetes, which are more similar to algae than fungi, which are more closely related to the animal kingdome.
The symptomes of the pythium species are typically associated with damping-off, which results in weak and fragile cuttings that fall over and exhibit browned / mushy roots (Zamir K. Punja & Gina Rodriguez; 2018).
P. aphanidermatum has also been reported to cause crown wilt (Beckerman et al.,2017)
They mostly spread via water contact, which their colloquial name, water molds, suggests. This spread is mediated by zoospores and mycelium, which upon contact, expands on the plant surface and subsequently penerates the cuticula.
Horizontal transmission is also the predominant way of spreading in a propagation. The oomcytes mostly infect young plants with adult plants showing infection rates of <5%.
Seed surface contamination is the most prominent way for seedling infection. (Zamir K. Punja & Gina Rodriguez; 2018).
Powdery mildew
The symptomes real powdery mildew is caused by different pathogens in different plant species. In Cannabis sativa the causal pathogen is of the genus Golovinomyces, detected in Cananda by (Pépin et al. 2018) belonging to the family of Ascomycetes.
The symptomes are mostly topologically and associated with the typical white mycelium on the leafs, which expressed conodia and haustoria which break through the cuticula into the plants mesophyll cells. (Cameron Scott & Zamir K. Punja, 2021)
Symptomes are characterised by the typical round, white mycelium on the leaf surface which spreads throughout the plant over its lifecycle. Also early flowering organs such as flower primordia are also susceptible, which are damaged by the fungus over its developement cycle severly impacting the reproductive capacities
The spread is also mostly achieved via horizontal transmission via Spores and mycelium contamination. Vertical transmission, again, mostly on the cutting surface or the seed surface
Botrytis cinerea
Botrytis cinerea, commonly known as gray mold, poses a serious threat to cannabis cultivation. This necrotrophic fungus thrives in cool and humid conditions, causing bud rot and other diseases. Infected cannabis plants display wilting, browning, and a characteristic gray mold on buds. The fungus spreads rapidly, particularly during flowering, leading to considerable crop losses. Control measures include maintaining optimal humidity levels, proper plant spacing for air circulat.