Hop Latent Viroid – A pandemic among cannabis and hops

In recent years, an invisible threat that is quietly and inexorably spreading has plagued the world of cannabis and hops: the Hop Latent Viroid. This tiny but devastating pathogen has spread silently through crop populations, causing enormous economic damage as well as serious ecological concerns. Hop Latent Viroid (HLVd) has become a true pandemic among cannabis and hops plants and its effects are far-reaching. For this reason, some researchers have dubbed the viroid the “COVID of the cannabis world.” In this article, we will take a closer look at Hop Latent Viroid, analyze its devastating effects on plants, and explore ways to prevent infections and combat this invisible enemy.

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Stunted growth after HLVd-Infektion – left: infected plant, right: healthy plant of the same age. Picture source: Université de Sherbrooke, Kanada[6]

What is Hop Latent Viroid?

Hop latent viroid (HLV, HLVd or HpLVd) is a circular RNA molecule which has an even simpler structure and is much smaller than a real virus. Just like a virus, it infects plants and reproduces inside them by forcing the infected cells to make copies of itself. As the name suggests, hop latent viroid was first discovered in hop cultures in the 1960s. At the latest since 2012 , it has also been detected in the closely related genus Cannabis[1].
In nature, HLVd spreads mainly through herbivorous insects such as aphids
[2] whose bites cause wounds that the viroid uses to enter the plant. In plant cultures it spreads primarily via unclean gardening tools or through propagation of cuttings from infected mother plants[3]. The pathogen can also be found in the seeds of plants, although studies show that the prevalence is significantly lower[2][5]. However, further research is needed on this, as a 2024 study observed a transmission rate of 58 – 84% through seeds[14].
The pathogen can remain dormant (latent) in a plant for a long time, up to several years, without killing it directly or causing visible symptoms
[4][14]. With increasing age, the amount of pathogens present increases as the strength of the immune defense is diminished in older plants. If the plant then experiences stress from the environment, the disease and all its effects become apparent.

How widespread is HLVd?

In 2021, a shock went through the world of cannabis breeders. A study by Dark Heart Nurseries analyzing around 200,000 tissue samples found that around 90% of all cannabis grows in California were infected with hop latent viroid[5][6]. According to estimates, around a third of all cannabis plants in the USA are infected with the pathogen. This disturbing statistic highlights the seriousness of the situation and shows the imminent threat to the hemp industry.
Unfortunately, the US is not the only country with this problem. The pathogen is spread worldwide and has long since become the most threatening disease to cannabis[6]. A study in the hop region Hallertau in Germany showed plants infected with the viroid as early as 1988[7]. According to a study on Polish hop fields, around 40% of the plants were infected in 2014[2]. Now this invisible epidemic continues to spread relentlessly in hemp plants. In the United States, farmers and breeders are already taking necessary routine measures to contain the spread and protect their crops. In Europe, where the cannabis industry is currently experiencing a rampant growth, it is now of utmost importance that cannabis growers improve their cultivation practices in order to remain competitive in the market.

Why is Hop Latent Viroid so harmful?

The threat posed by Hop Latent Viroid can have devastating effects on the economic cultivation of cannabis plants and hops. An infection with the viroid causes a dangerous chain reaction for plant growth: the infected plants are forced to reproduce the viroid and are thus weakened and more susceptible to other diseases[6] as well as stress factors. All organs of the plant are affected. In particular, reduced root growth impairs nutrient absorption, while reduced leaf growth reduces photosynthesis performance and thus also hinders the production of secondary metabolic products such as cannabinoids and terpenes.
The most well-known effect of the infection is the so-called “dudding disease“, which directly affects the quality and yield of the crops. Studies have shown that infection with HLVd results in stunted trichomes and can reduce the terpene and cannabinoid content of plants by up to 50-70%[6][8][9][10]. Terpenes are the aromatic compounds that determine the characteristic scent and taste of the plants, while cannabinoids such as THC and CBD are responsible for the medicinal and psychoactive properties. The reduction of these important compounds significantly affects not only the quality of plant products but also the economic viability of crops. Hop Latent Viroid is therefore one of the biggest threats to cannabis growers and farmers who want to extract CBD or other valuable substances from hemp.

Hop latent viroid Trichome. Auswirkungen auf Cannabinoid-Produktion: senkt THC Gehalt und CBD Gehalt

Electron microscope image of cannabis trichomes. The trichomes on the left are atrophied and smaller due to infection with Hop Latent Viroid. Right: healthy trichomes. Image source: Simon Fraser University, Canada

How can you recognize an infection with Hop latent viroid?

HLVd infection is difficult to detect as it often presents with subtle and nonspecific symptoms. In the vast majority of cases, no external symptoms can be observed in the first few weeks[14]. However, with more severe infestations, some of the following symptoms may occur:

  • altered leaf growth: This can manifest in the form of stunted leaves, irregular leaf shape, or atypical growth patterns. Some leaves grow smaller or deformed.
  • brittle stems
  • reduced root growth: This causes the rooting of cuttings taken from infected plants to be more difficult[11]. As another consequence, this growth inhibition also leads to reduced nutrient absorption.
  • yellowing of leaves (chlorosis): A common symptom of HLVd infections is yellowing of leaves. This often affects the youngest leaves that grow near the tips of the plant.
  • stunted plant growth: The plant is usually shorter with smaller leaves and generally smaller in stature.
  • irregular flower formation: Infection with HLVd can affect the development and formation of flowers. Infected plants may bear fewer flowers or have smaller or less vigorous flowers than healthy plants.
  • loss of aromas and flavor: sometimes recognizable by a weaker smell[12]: HLVd infection can lead to a loss of aromatic compounds such as terpenes. This can result in the plant’s characteristic scent and taste being affected.

Many of these symptoms overlap with other problems, such as nutrient deficiencies or other illnesses, which is why even experienced growers have difficulty identifying an infection with the naked eye. From a morphological point of view, infected plants are usually identical to healthy ones, but are significantly smaller overall, which only becomes noticeable later in their growth. To ensure that it is indeed an HLVd infection causing growth problems, a professional diagnosis through a laboratory test is essential. Only special diagnostic procedures such as the highly sensitive reverse transcriptase polymerase chain reaction (RT-PCR) can clearly detect the presence of HLVd with high accuracy.

How can I protect my plants?

Prevention is key when it comes to protecting your grow from Hop Latent Viroid[4][6]. Here are some tried and tested methods to minimize the risk of infection:

  1. Buy healthy plants: Start with healthy plants to minimize the risk of HLVd infection from the beginning. Choose certified plants that have been regularly tested for HLVd and other diseases. Avoid purchasing plants from unsafe or unknown sources as this could increase the risk of introducing an infection. Ask your local grow shop whether the clones you want to buy stem from a tested mother plant.
  2. Regular testing: knowledge is power. You can only prevent pathogens from spreading if you know the health status of your plants. Unfortunately, you can never eliminate the risk of introducing a pathogen into the grow facility completely. It is therefore advisable to regularly test the plants for the presence diseases. The optimal testing frequency depends on individual economic conditions of your operation. However, as a general guideline, professional companies should ideally conduct monthly testing of their most valuable cultivars. The earlier an infection is detected, the better the chances of fighting it successfully.
  3. Quarantine and monitoring: Place new plants in a temporary quarantine of approximately 30 days by keeping them separate from other plants[6]. During this period, you can carefully monitor and test the plants to ensure there are no signs of HLVd. Early detection is necessary for containment. Therefore, make sure the plants are not infected before entering your grow. Particularly valuable mother plants should be raised separately from the remaining cultivars if possible.
  4. Hygiene measures: Be sure to keep your growing area clean. Regularly remove dead plant debris and weeds to eliminate potential breeding grounds for insects and pathogens. Hop latent viroid can be detected in dried leaves for up to 4 weeks.
  5. Disinfecting tools: Thoroughly clean and disinfect gardening tools after each use. Unfortunately, due to the viroid’s resistant nature, simple disinfection is not enough. The viroid has been shown to be resistant to common antibacterial cleaners and alcohol.

So what works?

    • A solution of 10-20% bleach in water can kill the pathogens and prevent their transmission. Clean your tools for at least 60 seconds before working on the next plant.
    • Ribonuclease, or RNAse for short, is an enzyme that has the specific purpose of destroying RNA. Since Hop latent viroid consists entirely of RNA, it can be destroyed with RNAse. This remedy is guaranteed to work while not harming the plants. Disadvantage: High costs.
    • Heating the tools: At least at 160°C for a minimum of 10 minutes.
    • Virkon S: This special disinfectant against viruses and viroids is effective when used in a concentration of at least 2% (20g/L).[13].
  1. Avoiding a shared water cycle: The viroid can spread through the system should it enter the water. Therefore, avoid closed irrigation systems where several plants come into contact with the same water.
  2. Avoiding stressors: Keep your plants healthy by minimizing stressors such as extreme temperatures, inadequate watering, and nutrient deficiencies. If the plant is weakened by stress, its immune system cannot stop the viroid from multiplying and the disease can break out.

What do I do if my plants are infected?

Unfortunately, even the best hygiene protocols can fail. While the discovery of Hop Latent Viroid infection in your plant population is undoubtedly alarming news, the steps you take afterward can have a significant impact on the course of the infection. Here are some important actions you can take:

  1. Isolate and monitor: If you suspect your plants may be infected, isolate them from healthy plants immediately. This will prevent the infection from spreading further. Monitor infected plants closely to see how the infection develops.
  2. Remove infected plants: Once infection of a plant is confirmed, it is best to carefully remove and destroy the infected plants, as painful as it may be. This helps prevent the spreading of the infection to healthy plants.
  3. Meristematic tissue culture: Meristematic tissue culture is the only known method of saving infected plants[6]. The meristem is the apical (upper) growth tissue. It contains cells that are not yet fully differentiated and often have not yet taken up any pathogens. During the cultivation of meristematic tissue, tiny pieces of tissue are removed from this area and put in a special nutrient medium under sterile conditions. This complex process requires an extremely clean environment and can be carried out by specialized laboratories. Although the success of this method is never 100%, it is sometimes the only way to save an infected cultivar.

In summary

Hop Latent Viroid is an invisible threat that is spreading quietly and poses a major threat to cannabis and hop plantations. The consequences of HLVd infection can be devastating, from reduced crop yields to impaired quality of cannabis and hops products. Prevention is key to controlling the disease and requires the knowledge and collaboration of farmers, gardeners and researchers. By implementing best hygiene practices, monitoring and early detection of symptoms, you can help mitigate the dangers of this invisible pandemic and ensure the quality of products in the cannabis market.

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