Propagation of Hop Latent Viroid: the great silent threat to cannabis

Cannabis cultivation faces many challenges, one of which is the threat of diseases caused by pathogens such as the Hop Latent Viroid. This is a small molecular parasite that is highly infectious and is responsible for enormous economic damage, as it causes a reduction in the yielding capacity and quality of crops. Here we will delve into its complexities, modes of transmission, and the control strategies that can help to mitigate its harmful effects.

Hop latent viroid (HLVd or HpLVd) is a small infectious RNA molecule that affects hop plants (Humulus lupulus), a species that is first cousin to cannabis (both belong to the Cannabinaceae family) and which is mainly grown to produce beer. HLVd belongs to the Avsunviroidae family and is considered a viroid, i.e., a pathogen consisting of only an RNA molecule without a protein cover.

Viroids are miniature infectious agents that are composed solely of genetic material. Unlike viruses, whose genome is encapsulated in a protective layer, viroids are made up of bare RNA, so they are assumed to be unstable and cannot remain infectious outside the host for long. Just like DNA, RNA contains all the information necessary for a viroid to replicate within its host, so they depend entirely on it for propagation, which means that it has no known effect on human health.

HLVd was first identified in the 1970s and has since been found in hop-producing regions around the world. It is not known exactly when HLVd jumped over to cannabis plants, but the most plausible explanation is that it is the result of attempts to cross both plant species. However, given the prevalence of the hop latent viroid, it is likely that this pathogen has been spreading in cannabis for quite some time.

HLVd can infect plants without causing any noticeable symptoms (i.e., latent infections). But, under certain conditions, such as stress or co-infection with other pathogens, this viroid can be activated and cause the development of various diseases, leading to stunted growth, reduced yielding capacity, poor flower quality, leaf discoloration or loss, or rapid death of the plants.

Impact of HLVd in cannabis growing

In 2017, several producers in California reported stunted growth in multiple Cannabis sativa plantations. However, no other distinctive symptoms were observed to conclude what affected them, until later scientific analysis showed that they were infected with HLVd.

Currently, it is estimated that 73% of cannabis crops worldwide are affected by this viroid. Surprisingly, there are reports of up to 90% of crops testing positive for HLVd in places like California. This is a daunting statistic for an industry still in its formative years.

From a financial perspective, this prevalence rate translates into approximately $4 billion in annual losses, resulting from reduced yielding capacity and detection, control, and eradication costs. To further aggravate these losses, the impact of this viroid on the cannabis flowers can lead to a substantial 30-50% reduction in cannabinoid content, which can significantly decrease the commercial value of production.

What are the symptoms of Hop Latent Viroid?

The symptoms produced by HLVd vary widely, depending on the stage of development that the plant is at when infection occurs:

• Propagation stage: Root length is greatly reduced in cuttings taken from HLVd-infected mother plants, resulting in low-quality rooted plants that are more susceptible to root-attacking pathogens such as Fusarium and Pythium.
• Vegetative phase: Plants infected during this stage will often show stunted growth, including brittle stems; smaller, narrower, discoloured, or malformed leaves; shorter internodes; and even a similar smell to that of composting leaves.
• Flowering stage: Signs of infection are often more evident, and plants usually grow smaller than healthy plants. The leaves near the flowering buds may also exhibit an unexpected yellowish colour. And not only will they produce smaller flowers but also underdeveloped trichomes that look like a deflated balloon rather than their normal ball shape.

How is Hop Latent Viroid transmitted?

Transmission of HLVd in cannabis can occur through several channels. Once infection occurs, it can spread throughout the plant, affecting different tissues and organs:

• Transmission via infected cuttings. This is the most likely source of propagation. When a cutting is taken from an infected mother plant, several studies have observed a 100% incidence of Hop Latent Viroid in the rooting of these cuttings. Therefore, roots are a great place to diagnose HLVd.
• Transmission by infected seeds. The second most likely source of transmission is through seeds harvested from infected plants. This can occur at frequencies from 5 to 35%, depending on the level of initial infection in the parent plant. Even if you have an infected mother plant but the pollen is clean, many of the seeds (in some cases up to 30-40%) will carry the viroid inside or on the shell.
• Mechanical transmission. The third most likely source of propagation is through infected sap in contaminated tools used to prune or defoliate plants or to obtain cuttings. The spread of HLVd goes from the cut stem (an open wound) to the roots, and then propagates from the roots to the rest of the plant. The maximum infection rate is approximately 25% for any cut, pruned, or damaged surface that allows this viroid to enter.
• Waterborne transmission. While there is no scientific evidence (yet) that HLVd spreads simply when the leaves of plants touch each other, root-to-root contact or fertigation methods involving recirculation of water or nutrient solutions is a fourth source of propagation. Studies have highlighted that viroids are noticeably concentrated in the roots of the plants and can migrate to water through the runoff, with an occurrence rate of about 20%.
• Transmission by insects and other vectors. Lastly, although it is not yet known whether insects play a direct role in the transmission of HLVd, many viroids are known to spread through vector insects. Moreover, recent findings suggest that certain pathogenic fungi (including Fusarium) may play a role in viroid transmission.

Is Hop Latent Viroid systemic?

HLVd moves systemically all over the plant through the phloem (i.e., the conductive tissue responsible for transporting nutrients) for a period of approximately 6 weeks: it enters the phloem at the point of infection, from where it travels to the roots and then throughout the plant. Experiments have shown that:

• 2 weeks after infection, HLVd can be detected in the roots.
• 4 weeks after infection, HLVd can be detected on young leaves.
• 6 weeks after infection, HLVd can be detected all over the plant.

This means that, if you test a plant less than 6 weeks after infection, some tissues will be positive for HLVd, whereas others will be negative. That’s why it is important to examine several parts of the plant when looking for signs of infection.

How to get rid of the Hop Latent Viroid

It takes time, but the best method to control this viroid in a grow is through the process of testing and eliminating the infected plants. Meristem tissue culture can also be used to produce viroid-free clones, but this is a long and arduous process.

Therefore, as with most plant pathogens, prevention is key if you want to avoid this ‘silent killer’ altogether. This means that growers must adopt a multifaceted approach:

• Sterilisation: Regular sterilisation of tools and equipment (especially those used for pruning or trimming) is essential, as HLVd can remain infectious for up to 5 days on nitrile gloves, up to 8 days on tools, and up to 4 weeks in dry leaves. Using a 10% bleach solution to clean gloves and tools can significantly reduce transmission risks (heat and alcohol are not effective in killing this viroid).
• Quarantine: Isolating new plants or cuttings over a certain period, and closely monitoring them for any symptoms, can help prevent the introduction of HLVd in a cannabis grow.
• Testing and monitoring: Hop latent viroid cannot be diagnosed based solely on symptoms. The only way to determine if a plant is infected is through a diagnostic test, such as a PCR (polymerase chain reaction), which is usually considered the most accurate test to help with early detection.
• Genetic improvement: Although there are currently no HLVd-resistant cannabis cultivars, future research could shift in this direction, targeting strains that naturally resist this viroid. This is the case for hops, where HLVd is widespread, but does not cause significant economic damage to the crop.
• Personal protective measures: People handling plants should wear protective clothing and gloves. Since HLVd can remain infectious on human skin for approximately three hours, and for over 24 hours on materials such as cotton and leather, strict compliance with hygiene standards is essential.

Hop latent viroid represents a major threat to the cannabis industry and causes substantial economic losses. A proactive approach to its prevention is essential to minimise its impact. Regular testing, proper disinfection protocols, and adherence to pathogen prevention programs can all help ensure the health and vitality of cannabis crops in the face of this global pandemic, which has already been dubbed ‘the COVID of marijuana’.