Everything you need to know about Hop latent viroid

Hop latent viroid (HLVd, HLV or HpLVd) is a tiny, molecular parasite that is infectious in plants - sometimes known as ‘dudding disease.’ HLVd is the molecular cause of “dudding” in cannabis plants (1,2). Major economic losses are attributed to hop latent viroid due to this pathogen’s negative impact on flower quality. Estimates indicate +20% total accumulated production loss and some of our customers have sustained infections in 50%-60% of their grow rooms. Management of hop latent viroid is a critical consideration for all cannabis cultivators.

Looking for some quick facts on HLVd? Read What is HLVd?

1. What is a viroid?

Viroids are miniature infectious agents that are composed of only genetic material. As opposed to viruses, whose genome is encapsulated by a protective layer, viroids are naked RNA. This RNA, just like DNA, contains all the information needed for a viroid to replicate inside its host. Because viroids are so simple, they are completely dependent on the host plant to allow replication and spread.

Mature Viroid

2. How stable is a viroid?

Viroids are naked RNA so it is often assumed they are unstable and cannot remain infectious outside of a host for long. This assumption is based on the relatively fragile nature of the average RNA molecule. However, viroids are not average RNA molecules. They are rod-shaped molecules, tightly bound to themselves, like two pieces of velcro.

The hop latent viroid structure, called double-stranded RNA, is very stable and rigid, allowing viroids to persist even in harsh environments. Studies have shown that viroids can remain infectious for longer than 24 hours on most common surfaces (3), 7 weeks in water (4) and months to years in dried plant debris and seeds (5). Careful and intentional sterilization is critical to limit viroid spread.

3. Where did Hop latent viroid originate?

Hop latent viroid (HLVd) was first characterized in hop plants (the plants that give the bitter taste to beer) in 1987 (6).  In hops, HLVd is largely asymptomatic, which has allowed this pathogen to spread globally. Hop latent viroid has been identified on most continents in the world and in some fields more than 90% of hop plants are infected (7). Despite this high rate of infection, few containment measures are in place for hop latent viroid in hops because the infection does not cause serious economic consequences in this crop. Exactly when HLVd cross-species jumped into cannabis is not known. However, based on the prevalence of hop latent viroid, this pathogen has likely been spreading in cannabis for quite some time.

4. Which plants can be infected by Hop latent viroid?

Hop latent viroid has a fairly narrow host range. The only plants characterized to be natural hosts of HLVd are hops plants, stinging nettle, and cannabis. However, viroids are capable of rapid mutation and evolution, so it is possible that an alternative HLVd sequence could evolve to infect other crops. In line with this idea, it is known that heat treatment results in mutations in the HLVd genome that could allow the viroid to replicate in other crops (8).

5. How does Hop latent viroid spread and replicate?

Spread

Mechanical Transmission

Hop latent viroid is primarily spread mechanically. For the infection to spread, sap from an infected plant must gain access to a wound or microtear on a healthy plant. The most common way viroids spread in a facility is during plant trimming. If tools are not properly sterilized after trimming an infected plant, those tools will readily spread HLVd to healthy plants.

While touching can also spread hop latent viroid, this method of transmission is less common. If there is sap or plant debris from a sick plant on the hands of an individual, rough touching of other plants that results in a micro-tear or abrasion can spread HLVd. Sanitation of gloved hands between plants is highly recommended.

Seed Transmission

While no published studies exist in cannabis describing the frequency of seed transmission, HLVd does transmit through seeds in hop plants at a rate of around 8% (9). TUMI Genomics is currently conducting a research collaboration to determine the percent seed transmission of HLVd in cannabis- stay tuned.

Water Transmission

Water transmission is another potential route of spreading. Viroids are in high concentration in the roots (10) and can move from the root into runoff water (11). Water contaminated with high levels of viroid can spread the infection to healthy plants. If recirculating water or any shared water sources are utilized, water transmission is a potential concern.

Insect Transmission

There are currently no published reports showing insect transmission of hop latent viroid. However, numerous other viroids are spread by insects, making insect transmission of HLVd very likely. Regardless, limiting insects’ access to plants should be standard practice, as many other viruses and pathogens are spread by insects.

Recent reports also suggest that pathogenic fungi, such as Fusarium, could support viroid replication and spread the infection to host plants. While pathogenic fungus is a major concern for cannabis growers, limiting the prevalence and spread of fungal pathogens could help limit hop latent viroid transmission as well. Mechanical, seed, water, and insect transmission are just a few methods of viroid transmission. For more insights on other potential transmission methods, read How Does Hop latent viroid spread between cannabis plants?

Replicate

Viroids are intracellular parasites that are completely dependent on the plant for replication. Viroids like HLVd exist mostly inside the cells of an infected plant, in a compartment called the nucleus. In the nucleus, HLVd changes gene expression forcing the molecular machine that interprets the DNA (called RNA Polymerase II) to instead replicate the viroid. Because of the location of HLVd in infected plants, non-specific disinfectants, either applied topically or used to dip a clone, cannot cure hop latent viroid. Any disinfectant that indiscriminately destroys RNA, inside living cells, would also kill the plant.

Example of systemic infection moving through the vascular system of a cannabis plant

Hop latent viroid establishes a systemic infection by moving through the vascular system of the plant. Because viroids cannot traffic independently, they are dependent on the normal movement of sap to transport them from the site of infection to new tissue. Because of this, viroids do not infect all parts of a plant evenly. Sap containing energy generated in leaf tissue (aka photoassimilates) is trafficked to areas of active growth in the plant, called the “source to sink'' movement.

When the plant is infected viroids will travel more readily to areas actively growing - as well as to the roots - which require energy generated from foliage to grow. This means that not every branch in an infected plant will initially contain hop latent viroid. This means a sampling of multiple locations around a plant is critical to correctly diagnose HLVd.

6. What are Hop latent viroid symptoms?

Hop latent viroid has very pleiotropic effects, meaning the symptoms can range from severe to extremely subtle. An HLVd infection can impact growth patterns, leaf shape, and flower content and quality.

However, Hop latent viroid symptoms aren’t always obvious. HLVd can also present asymptomatically, especially in newly infected plants or plants in a vegetative state. While symptoms may not be apparent, these plants can still spread the infection to healthy plants in your facility. HLVd cannot be diagnosed based on symptoms alone. The only way to be sure a plant is infected or not is through a molecular diagnostic test.

Images from Dark Heart Nursery and Cannabis Business Times

7. How do I prevent Hop latent viroid in my plants?

The single most effective way to keep your facility safe from hop latent viroid is prevention. There is no topical hop latent viroid treatment or cure. While critical varieties can be cleaned via a lengthy tissue culture process, nothing can rescue a run failing due to hop latent viroid. At TUMI Genomics we have created a pathogen prevention program, S.T.O.P., to help growers take intelligent steps to protect their plants from pathogens, including HLVd. The S.T.O.P. program guide is available for download.

8. How should I disinfect tools and equipment from Hop latent viroid?

Most disinfectants are effective against pathogens that are composed in part by proteins and/or fat, like fungi, bacteria, and viruses. However, the hop latent viroid structure, and viroids in general, prevents common disinfectants from destroying hop latent viroid.

Household bleach is the most effective disinfectant shown to destroy viroids. Tools should be soaked in a 10% bleach solution to prevent or remove viroid contamination. This equals 1 part bleach mixed with 9 parts water for 60 seconds in between each plant. Be sure to select a bleach brand whose main ingredient (sodium hypochlorite) is 6% or greater. Alternating between several pairs of scissors in a bleach solution can make disinfection efficient during trimming.

Common solutions that DO NOT eliminate viroids include alcohol and hydrogen peroxide. Heat or UV sterilization is also not 100% effective. For more information, download our review comparing common HLVD sterilization methods.

9. Is Hop latent viroid testing available? How often should I sample plants for Hop latent viroid testing?

Hop latent viroid cannot be diagnosed based on symptoms alone. The only way to determine if a plant is infected is through a diagnostic test. Due to the prevalence and potential economic loss from this pathogen, regular pathogen testing of mother plants every 4-6 weeks is recommended.

For insights on Hop latent testing on-site or in-lab, read Hop Latent Viroid testing: On-site or in-lab?

HLVd and all viroids can only be detected by a molecular test (a test that detects the presence of DNA/RNA). Among common molecular tests, PCR is generally the most sensitive and accurate method. PCR can provide a diagnosis and an approximate viroid level, allowing informed management decisions. Other types of molecular tests, such as LAMP (Loop-mediated isothermal amplification) and RPA (Recombinase polymerase amplification), can formally be as sensitive as PCR. But, the classic versions of these assays often suffer from false positive/negative results, reducing accuracy. For more insights on the strengths and weaknesses of different HLVD detection techniques, read Hop latent viroid: A review of cannabis testing methods.

While various types of in-lab tests are available, PCR is the most sensitive and accurate. At TUMI Genomics, we employ a specialized type of PCR called TaqMan-qPCR, which has increased sensitivity and specificity over standard PCR. For information about our HLVd test accuracy, download our validation document.

10. How do I sample my plants for Hop latent viroid testing?

Viroids do not move about the plant evenly in all tissues. A recent collaboration with Kaprikorn Oregon Craft Cannabis concluded that root tissue is the most reliable tissue for detection of HLVd. Find the details in our research study download.

For the most accurate and sterile sample collection, we have comprehensive instructions available here.

Stop a Hop latent viroid infection before it starts

Hop latent viroid poses a significant threat to the cannabis industry, leading to substantial economic losses. A proactive approach to HLVd prevention is essential for minimizing the infection impact. Regular testing, proper disinfection protocols, and adherence to pathogen prevention programs can help ensure the health and vitality of cannabis crops in the face of this global pandemic.

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About TUMI Genomics

TUMI Genomics is advancing cannabis cultivation by bringing biological insights to help commercial growers thrive. Through comprehensive preventative solutions, superior pathogen diagnostics, and AI-driven insights, TUMI Genomics is helping cultivators start clean and stay clean. The unrivaled diagnostics, transparent validation and industry-leading pathogen detection innovations provided by TUMI Genomics are raising the standards of commercial cannabis cultivation. The team at TUMI Genomics is composed of deeply experienced Ph.D. molecular biologists, bioinformaticians, business strategists, and accomplished serial entrepreneurs.

Media Contact: Hailey Spencer, Marketing Director

hailey@tumigenomics.com

Research Inquiries: Tassa Saldi, Ph.D., Co-Founder and CSO

tassa@tumigenomics.com