Hop Latent Viroid is a Major Threat to the Cannabis Industry, Review Claims
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Hop latent viroid (HLVd) infection is a major concern for cannabis cultivators worldwide. This is the warning espoused in a new review article from researchers at the Université de Sherbrooke, Quebec, and Hirosaki University, Japan.
Published in the journal Viruses, the review details the discovery of HLVd in hops and subsequently in the cannabis plant, provides a brief overview of the extent of these infections in the industry, and offers guidance on the prevention and management of infections.
Additionally, the researchers also identify areas for future research that will help to expand the knowledge base surrounding HLVd and cannabis, as well as key priorities for development in order to tackle the impacts of HLVd.
What is HLVd?
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.
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.
As the new review summarizes, studies so far have found very little to no evidence suggesting that HLVd spread 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.
What does HLVd mean for cannabis?
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.
Interestingly, the review article also notes that 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. Although, as the review authors also point out, there is no current evidence of any HLVd-resistant cultivars.
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.
Preventing the spread of viroid species
In 2021, experts from a cannabis nursery in California conducted more than 200,000 tissue tests oncannabis 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 on the proper steps to control and manage this viral infection.
As the review authors outline, 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, they recommend, with testing done in the third week of quarantine. Since viroid distribution is likely to be uneven in the plant, the review authors also recommend that multiple leaf samples from different stem heights be taken and tested.
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. As the review authors detail, 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. The review authors 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 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, they argue, should also be investigated.