Hop Latent Viroid: A Hidden Threat to the Cannabis Industry

Hop Latent Viroid (HLVd) is a plant-specific pathogenic RNA that poses a significant threat to the cannabis industry. In this comprehensive article, we will explore the nature of this infectious plant disease, its known symptoms, and how to prevent transmission in your cannabis grow operation.

What is Hop Latent Viroid (HLVd)?​

HLVd is a single-stranded, circular infectious RNA that is completely dependent on its host plant's metabolism for replication. As the name suggests, HLVd occurs worldwide in hops but it can also infect its relative, cannabis. Viroids are the smallest known agents of infectious disease, with HLVd being 50-80 times smaller than the smallest viral genomes. Unlike viruses, viroids do not have a protective protein coat.

The Discovery of Hop Latent Viroid​

The story of Hop Latent Viroid began in the late 1980s when researchers from the Institute of Agronomy and Food Technology in Spain reported the discovery of viroid-like RNA in two commercial varieties of hops grown in the Leon region of Spain. This viroid-like RNA did not seem to induce any negative disease symptoms in the hops, and so it was named the "hop latent viroid" (HLVd).

Later, studies on hops grown in the United Kingdom found that while HLVd-infected hop plants appear symptomless, this infection can actually significantly reduce the plants' yield as well as the levels of α-bitter acid or the essential oil content in the hop cone.

In 2019, the first research groups reported the detection of HLVd in cannabis plants in the USA. HLVd in cannabis is also commonly referred to as "dudding" or "dudding disease", with symptomatic infected plants generally growing with less vigor.

Symptoms of HLVd Infection in Cannabis Plants​

HLVd causes noticeable symptoms in cannabis plants, including:

• Growth stunting
• Brittle stems
• Reduced trichomes and oil production
• Reduced flower mass
• Malformation and/or chlorosis of the leaves

Interestingly, only a few cannabis cultivars actually show symptoms associated with HLVd when infected, which implies that the disease severity and symptoms could be genotype-dependent. In susceptible plants, HLVd can cause cannabis plants to have smaller leaves, stunted growth, malformations, yellowed leaves, and a reduced flower mass and trichomes. The buds of infected plants also tend to be smaller and looser, with less trichome production.

Crucially for the cannabis industry, HLVd can cause a 50% reduction in cannabinoid and terpene production.

Prevalence of HLVd in the Cannabis Industry​

In 2021, experts from a cannabis nursery in California conducted more than 200,000 tissue tests on cannabis from facilities across the state, finding that approximately 90% of facilities were testing positive for HLVd. According to the review authors, around 30% of the plants in each facility showed symptoms of infection.

Given the prevalence of HLVd in the cannabis industry already, it is important that cultivators are informed about the proper steps to control and manage this viral infection.

How does Hop Latent Viroid Spread?​

HLVd is most commonly spread via infected tools and equipment, which is why cultivators should always sterilize their equipment before starting work on a new plant.

HLVd can also spread through cloning when cuttings are taken from an infected mother. And because symptoms of HLVd are not always obvious in the vegetative stage, it can be hard to identify infected mother plants. This is especially true when infection occurs later in the plant's development since stunted growth will not be as apparent.

How to Prevent and Control Hop Latent Viroid​

Although HLVd can be eliminated from a cannabis or hemp plant via tissue culture, it is a long and laborious process that should only be reserved for cultivars that are critical to your business.

As with most plant pathogens, prevention is key. Good sanitation practices will go a long way in preventing the spread of HLVd and all other plant pathogens. Use fresh gloves each time you handle a new plant and sterilize tools often. Visitors and staff should also use footbaths before entering the growing area and wear hairnets, beard nets, gloves, and coveralls.

Cultivators should also screen mother plants with qPCR assays to ensure they are virus-free before taking any cuttings. Cultivators should also screen incoming clones with qPCR assays to make sure they are not introducing infected plants to their growing area.

Testing for Hop Latent Viroid​

Medicinal Genomics has developed a PathoSEEK® qPCR assay that can be used to screen plants for three common virus infections: Lettuce Chlorosis Virus, Hop Latent Viroid, and Cannabis Cryptic Virus. DNA can be extracted from infected fan leaves using Leaf Punch Lysis Solution and the multiplex assay can be run on the Agilent AriaMX and BioRad CFX96. If you are looking for a lab to test for HLVd, you can fill out a form and be connected with a partner lab in your area.

The Importance of Quarantine and Inspection​

It is essential to quarantine and carefully inspect any new varieties you receive before adding them to the rest of your plant stock. Plants and materials should be quarantined for around 30 days, with testing done in the third week of quarantine. Since viroid distribution is likely to be uneven in the plant, multiple leaf samples from different stem heights should be taken and tested.

Meristem Tissue Culture Techniques for HLVd Infections​

While there is no treatment available for treating HLVd infections, meristem tissue culture techniques can be used to save uninfected materials from HLVd-infected plants. However, while the meristem technique does produce virus-free plants, these plants are not viroid-resistant.

As a result, having strict preventative measures in place against viroid infection is key for cannabis plant cultivation facilities.

Future Research on Hop Latent Viroid​

Scientists have identified two distinct HLVd variants in cannabis species to date, which differ by a single genetic mutation. Further studies looking at these variants are still needed in order to understand whether or not both of the HLVd sequence variants are able to infect and induce disease symptoms in cannabis plants.

The review authors also recommend that, for sustainability reasons, it is important to find more practical long-term solutions to controlling HLVd infection. While appropriate chemical sterilization and meristem tissue culture propagation can tackle cases of infection, this can be laborious and expensive for cultivators. Longer-term solutions such as the possibility of breeding HLVd-resistant plant cultivars should also be investigated.

Conclusion​

Hop Latent Viroid is a hidden yet significant threat to the cannabis industry. By understanding the nature of this infectious plant disease, recognizing the symptoms, and implementing proper preventative measures, cannabis cultivators can effectively control and manage HLVd infections in their operations. Continued research is necessary to develop more practical long-term solutions and to explore the possibility of breeding HLVd-resistant cannabis cultivars.