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Hop Latent Viroid in Cannabis Cultivation
Hop latent viroid infection is a major concern for cannabis cultivators worldwide.
Hop latent viroid (HLVd) has emerged as a major challenge for cannabis cultivation and plant pathology research. A member of the viroid family, HLVd is one of the smallest known infectious agents, consisting of a single-stranded circular RNA molecule without a protective protein coat. Despite its simplicity, HLVd can severely compromise plant health and productivity, particularly in cannabis – an industry that relies heavily on consistent yields, cannabinoid profiles and terpene content.
The study of hop latent viroid is important not only for its economic impact on the cannabis and hop industries but also for advancing our understanding of viroid biology, plant–pathogen interactions and molecular detection technologies. This article explores hop latent viroid in cannabis, its discovery and spread, the consequences for cultivation and the diagnostic and management strategies available to mitigate its impact.
What is hop latent viroid?
Defining viroids
Viroids are a class of plant-infecting pathogens and are the smallest known agents of infectious disease. These single-stranded circles of RNA are typically 50-80 times smaller than the smallest viral genomes, and unlike viruses, viroids do not have a protective protein coat.
Discovery in hops
In the late 1980s, researchers from the Institute of Agronomy and Food Technology, Spain, reported the discovery of viroid-like RNA in two commercial varieties of hops grown in the León region of Spain. Notably, 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.
Transmission pathways
Studies so far have found very little to no evidence suggesting that HLVd spreads via pollen or seed, respectively. However, the viroid can be transmitted over long distances via the use of infected propagative materials, as well as spreading mechanically by grafting and through the use of contaminated tools or machinery.
Hop latent viroid in cannabis
Emergence in cannabis cultivation
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 in susceptible cultivars
Interestingly, only a few cannabis cultivars show symptoms associated with HLVd when infected, which implies that the disease severity and symptoms could be genotype-dependent. Although there is no current evidence of any HLVd-resistant cultivars. Symptoms can include:
- Smaller, yellowed, or malformed leaves
- Stunted growth and reduced plant vigor
- Looser, smaller buds with fewer trichomes
- Up to a 50% reduction in cannabinoid and terpene content
Industry impact
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 and 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 on the proper steps to control and manage this viral infection.
HLVd detection methods: RT-qPCR, sequencing and tissue culture
Accurate detection is central to managing hop latent viroid in cannabis. Several molecular biology tools are employed:
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RT-qPCR for viroid detection
- How it works: RNA is reverse-transcribed into complementary DNA (cDNA), which is then amplified using sequence-specific primers. Fluorescent probes track amplification in real time, allowing quantitative detection.
- Advantages: High sensitivity, specificity and rapid turnaround.
- Limitations: Requires specialized instrumentation and trained personnel.
Next-generation sequencing for viroid research
- How it works: High-throughput sequencing generates comprehensive profiles of plant transcriptomes, enabling viroid detection without prior sequence knowledge.
- Advantages: Detects known and novel viroids, provides genomic insights.
- Limitations: Cost-intensive, requires bioinformatics expertise.
Tissue culture testing in cannabis
- How it works: Small plant samples are cultured under sterile conditions, followed by viroid testing using molecular assays.
- Advantages: Useful for screening mother plants and propagation material.
- Limitations: Time-consuming, labor-intensive, requires controlled laboratory conditions.
Emerging plant viroid diagnostics
While antibodies cannot directly detect RNA-based viroids, hybridization-based assays and CRISPR-based diagnostic methods are being explored for faster and field-deployable detection.
Preventing the spread of viroid species
The first step of an effective prevention protocol is to test all of the cannabis plants and propagative materials coming into the facility, to ensure that they are viroid-free. Plants and materials should be quarantined for around 30 days, researchers recommend, with testing done in the third week of quarantine. Since viroid distribution is likely to be uneven in the plant, it is recommended that multiple leaf samples from different stem heights be taken and tested.
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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. This means having a regular testing system, sanitizing all equipment and tools with either diluted bleach or biocidal disinfectant and swiftly destroying any infected material in order to halt the spread of detected infections.
What’s next for HLVd research?
Scientists have identified two distinct HLVd variants in cannabis species to date, which differ by a single genetic mutation. Researchers say that 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 same researchers 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.
Hop latent viroid represents a significant threat to cannabis cultivation and poses a unique challenge for plant pathologists and molecular diagnosticians. With its high prevalence, ability to spread through routine cultivation practices and lack of resistant cultivars, managing HLVd requires a combination of robust molecular testing, strict sanitation protocols and forward-looking breeding and research efforts.
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