How serious is Hop Latent Viroid for growing cannabis?

By Nebula Haze

Have you heard about Hop Latent Viroid infecting cannabis plants? Unfortunately, HLVd is a cannabis plant pathogen that infects cannabis plants and causes a problem known as dudding disease (also called “dudders” or “dud”) or Putative Cannabis Infectious Agent (PCIA). This viroid has recently been infecting commercial marijuana growers and shutting down entire weed farms. But what is it? Should home cannabis growers be worried about Hop Latent Viroid, and if your cannabis garden already has Hop Latent Viroid, what do you do to get rid of it? Get the answers today!

The good news for home cannabis growers is that starting with seeds (instead of clones) ensures your plants don’t get Hop Latent Viroid!

Compare a healthy cannabis plant vs a cannabis plant with hop latent viroid. Note: The plant on the right was confirmed HLVd-positive at the lab.

Another comparison of a healthy cannabis plant vs a plant with hop latent viroid. Source: Hop Latent Viroid: A Hidden Threat to the Cannabis Industry
Source: Hop Latent Viroid: A Hidden Threat to the Cannabis Industry

Symptoms of Hop Latent Viroid in Cannabis Plants

Plant Symptoms

  • stunted plants
  • long side branches that tend to sag
  • discoloration or yellowing on leaves
  • dark or glossy leaves
  • leaf or stem abnormalities
  • strange growth patterns
  • small leaves with overlapping blades
  • rubbery stems that tend to snap off easily
  • overall loss of vigor and hardiness
  • some infected plants are asymptomatic and never show any disease symptoms (but they can still spread the viroid!)

Bud symptoms

  • greatly reduced trichome production and smell
  • lower levels of secondary metabolites (cannabinoids and terpenes)
  • up to 50% reduction in THC levels (!!!)
  • significantly reduced flower yield / biomass
  • it’s too confusing to diagnose based on symptoms; you need lab testing to identify hop latent viroid

What a terrible ailment! Luckily, as a home cannabis grower, you have a lot more ability to prevent this pathogen than commercial growers. In fact, home growers that start from seeds essentially never get this viroid. Check out these pictures, and then keep reading to learn more!

A healthy cannabis plant’s bud.

A healthy cannabis plant. Source: Dark Heart Nursery
Source: Dark Heart Nursery <– This nursery is the one spearheading research into cannabis hop latent viroid. They deserve a debt of gratitude.

The following cannabis plant is infected with hop latent viroid, though is otherwise the same strain, age, and conditions as the above plant. Notice how this bud has far fewer trichomes, even though the age and genetics are the same. The difference is a common symptom of being infected by hop latent viroid.

Same strain and age, but infected with hop latent viroid

Cannabis plant infected with hops latent viroid (otherwise the same strain, age, and conditions) Source: Dark Heart Nursery
Source: Dark Heart Nursery

Here’s an example of cannabis leaves infected with hop latent viroid vs clean leaves. Notice how the leaves from infected plants are smaller, have some discoloration, and include lots of little malformed leaves.

Example of cannabis leaves infected with hop latent viroid vs clean leaves. Notice how the leaves from infected plants are smaller, have some discoloration, and include lots of little malformed leaves.
Source: Modern Canna

Often, infected plants start showing symptoms of a latent infection after a period of stress.

 

What is Hop Latent Viroid (HLVd or HpLVd ) and When Did it Start Infecting Cannabis Plants?

Hop Latent Viroid is a pathogen known as a “viroid” which is “a single-stranded RNA molecule that lacks a protein shell and replicates using the host plant’s enzymes”. It’s called “hop” because it was first identified on hop plants, and “latent” because it can infect a plant without immediately causing any symptoms. In fact, some cannabis plants may only have mild symptoms or be asymptomatic forever. Some strains or plants seem more susceptible than others.

Learn more about the hop latent viroid from ScienceDirect.

How is a viroid different from a virus?

A viroid is smaller, does not encode proteins (instead relies on the host plant’s enzymes to replicate), and consists of a single-stranded RNA molecule with no protein shell.

When was Hop Latent Viroid discovered and when did it start infecting cannabis plants?

I didn’t hear about Hops Latent Viroid until 2022 at a cannabis business conference, but its history in agriculture extends back to the 80s.

  • 1987 – Hops Latent Viroid was first identified in hops plants in Spain (hence the name).1 Note: The Hops plant (Humulus lupulus L.) is the only other member of the Cannabaceae plant family besides cannabis.
  • 2015-2017 – Some cannabis growers started noticing HLVd infecting their plants, though they didn’t know what was causing the unusual symptoms. Initially, some growers called the undefined condition “dudding” “dud” or “dudders” because they noticed certain plants were making “dud” branches. Other growers called it “Putative Cannabis Infectious Agent” (PCIA). Unfortunately, growers soon realized the symptoms were spreading rapidly between plants and throughout the cannabis industry.2 3
  • 2019 – First scientific report on Hop Latent Viroid by Dark Heart Nursery, where they published their lab results showing extensive infections among commercial cannabis growers.4
  • 2021 – Hop Latent Viroid spread rapidly. By September 2021, Dark Heart Nursery published the results of 200,000 tissue tests they conducted across California and estimated that 90% of commercial cannabis facilities in the state had at least some plants infected with Hope Latent Viroid.5
  • 2023 – In May 2023, another study came out documenting the latest research and professional information about how hop latent viroid is affecting the cannabis industry.6

The left cannabis plant in this picture is infected with Hops Latent Viroid (also called PCIA or “Putative Cannabis Infectious Agent” by some growers, which is why it’s labeled that way). Notice how the infected plant has long drooping side branches and is overall smaller with undersized leaves.

The left cannabis plant in this picture is infected with Hops Latent Viroid (also called PCIA by some growers, which is why it's labeled that way). Notice how the infected plant has long drooping side branches and is overall smaller with undersized leaves. Source: Dark Heart Nursery
Source: Dark Heart Nursery

 

How to Cannabis Plants Catch Hop Latent Viroid?

Hop Latent Viroid can only infect your cannabis plants if they get exposed to the pathogen. If they never encounter HLVd in their life, they will never get it.

So how do cannabis plants catch Hop Laten Viroid?

  • Mechanical injury plus contact between plants – Whenever there is some kind of physical injury to the cannabis plant that exposes its insides to the outside, you have a high chance of HLVd moving between plants. For example, if you accidentally snap a stem and then touch another plant that also has an injury. It’s kind of analogous to the idea of how blood will transfer diseases, but most especially if someone’s blood touches an open wound on someone else’s body.
  • General contact with infected plants – Exposure to cannabis plants infected with Hop Latent Viroid may also spread the disease, though it’s much less likely than if there is mechanical damage and contact between plants.
  • Scissors and shears – When cannabis growers remove leaves or prune an infected plant, then use the same tool on other plants, it can infect them. Like if a surgeon went from one infected patient to another without cleaning their tools. Another time that growers use scissors is when making clones, which is especially heartbreaking because just one infected plant could turn into a room full of infected clones because the grower used the same scissors without cleaning them between plants.
  • On grower’s hands  – Just like with scissors, if a grower is touching a plant and touches other plants it can spread the viroid, especially when breaking the “skin” (for example plucking leaves) and touching multiple plants, allowing the viroid to easily get inside the plant and make a home.
  • What about seeds or pollen? So far, there hasn’t been evidence of transmission through seeds or pollen. It seems to be spread primarily through mechanical means.

If you trim an infected cannabis plant and then use the same scissors on a living plant, it can spread hops latent viroid.

 

How to Find Out If Your Cannabis Garden Has Hop Latent Viroid?

The only way to know for sure whether your cannabis plants have hop latent viroid (or a virus) is to get plant tissues tested in a lab. Often you have the same disease in the grow room but with different symptoms. This makes it too confusing to diagnose based on symptoms. You need to use lab testing to identify the pathogen and know what you have for sure.

Lab testing is the only way to know for sure if your cannabis plant has hop latent viroid.

Lab testing is the only way to know for sure if your cannabis plant has hop latent viroid. Source: Dark Heart Nursery
Source: Dark Heart Nursery

Here are some of the most common viruses and viroids that have spread to cannabis plants.

  • Hop latent viroid <– Biggest problem in the cannabis industry currently
  • Hop stunt viroid
  • Beet curly top virus
  • Tobacco mosaic virus
  • Lettuce chlorosis virus (Learn more)
  • Cannabis cryptic virus
  • Arabis mosaic virus

How do I get treat Hop Latent Viroid in infected cannabis plants?

Unfortunately, the best thing you can do if your plant has hop latent viroid is to immediately toss it. If that’s a choice, that’s your best one. However, what if you have a prized cultivar that simply can’t be thrown away?

In that case, the only reliable way to clean and reclaim your genetics is to go through an extensive process known as plant tissue culture or micropropagation. Essentially, you cut off tiny pieces of your cannabis plant, clean/treat the pieces, identify tissue that is viroid-free, and then grow the tiny plant pieces in test tubes into full plants. After that, the new plants are tested to ensure they’re clean, at which point they can be re-introduced to a clean grow space. Typically cannabis growers will get this done through an experienced lab as this can be extremely tough to do at home without specialized equipment.

Plant tissue culture is the only way to reclaim cannabis genetics if you have no healthy plants left.

Plant tissue culture is the only way to reclaim cannabis genetics if you have no healthy plants left. Source: Dark Heart Nursery
Source: Dark Heart Nursery

The reason plant tissue culture works is that the growing tips of the plant (where “meristem” cells are located) are often viroid free. After viroid-free pieces of the cannabis plant are identified, lab technicians grow those tiny pieces into full plants. Essentially, it’s like technicians are starting with a really, really tiny clone that needs a ton of special care to get it to grow into a full plant. This process of also called “micropropagation” because you’re taking a “micro” piece of pant and propagating it into a full plant.

It’s amazing that this tiny healthy cannabis plant piece can grow into a full plant!

Source: Dark Heart Nursery

 

How to Prevent Hop Latent Viroid from Infecting Cannabis?

The motto for a healthy grow room: “Start Clean and Stay Clean!”

  • Start with seeds. Never start growing with clones from another grow space if you can avoid it. Clones are by far the #1 reason plants get viroids, viruses, or pests that are hard to get rid of. Seeds are typically free of pathogens. As long as you don’t have hop latent viroid on your hands, and the seed doesn’t have hop latent viroid on the shell, the resulting plant won’t catch it. If worried, treat seeds with a brief dip in H2O2. This will kill any viroids or pathogens on them. Wash your hands and wear gloves after that when you touch the seeds. Your cannabis plants will be viroid-free even if their parents were somehow infected (which is very unlikely if you are buying seeds from a professional breeder)
  • Test plants often – If you must use clones, make sure to regularly test your plants. Don’t wait until you have symptoms! When you get new clones or plants from a different grow space, quarantine them away from your other plants for at least 3 weeks to ensure they seem healthy and don’t have any unwanted critters hopping a ride that could be carrying HLVd. Get these clones tested at a lab. Only after you’re sure the clone is virus, viroid, and pest-free should you introduce it into your grow room. But as a home grower, your best bet for a clean grow room is to always start with seeds, saving you a lot of time and trouble.
  • Don’t ignore bugs. Bugs carry diseases and spread them from plant to plant, especially aphids, white flies, thrips, and leaf hoppers. So if you get a clone that has a viroid and then get a bug infestation, the bugs can spread the viroid to all your plants even if you’re careful not to let the plants touch each other. Bugs from another grow room can also bring in viroids.
  • Good environment  – A good environment is not only the best way to prevent mold, bud rot, or white powdery mildew, but it also helps prevent bugs and pests, which can carry viroids. Pests tend to proliferate in hot or stagnant growing environments. What to pay attention to: ensure there is good air circulation, avoid high temperatures, and ensure the air humidity never gets high. For home growers, keeping plants in a grow tent with a strong exhaust fan is typically all you need to make a great breezy growing environment.
  • Remove infected plants immediately – The moment you identify an infected plant, carefully remove it from the grow space and safely get rid of it. Don’t just cut off sick branches or wait until they’re ready to harvest.

It makes me glad to be a home grower. I can’t imagine the heartache of having to toss dozens of plants due to a pest outbreak that came from an infested clone.

Start with seeds to avoid accidentally introducing cannabis diseases or bugs to your grow room.

 

References

I also learned a lot from other growers about hop latent viroid at the Cannabis Business Conference in 2022 (the conference hosted an hour-long presentation on it).

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