Plant Talk: Hop Latent Viroid

Nov 5, 2021 | Blog, News

The hop latent viroid is keeping cannabis growers awake at night. They worry that HLVd will not only cut into profits but decimate entire crops and eradicate prized heirloom varietals. As HLVd spreads across the country, and possibly the world, researchers are working overtime to figure out what it is, how it affects the plants, and most of all, how to stop it. Oaksterdam University Horticulture Expert Jeff Jones is on the forefront of the battle against HLVd. As questions about the viroid crop up in OU’s classrooms, Friday Horticulture Labs, and Clubhouse chats, he shares some insight into the disease that’s threatening the industry. 

A plant with HLVd (labeled PCIA for putative cannabis infectious agents) vs. a healthy plant. Photo by Dark Heart Nursery.

What is Hop Latent Viroid?

Viroids are small pathogens that infect plants, composed of circular, single-stranded RNA. Unlike viruses, they have no protein coating. HLVd is so named because it occurs worldwide in hops, but it also affects hops’ sister plant, cannabis.

What are the symptoms?

Cannabis plants infected with HLVd start out completely fine — hence the word “latent” in its name. Growth is normal and everything seems to be on track. Then, at the flowering stage, something goes wrong. Plants start “dudding out.” They become stunted with smaller, spiky leaves. Nodes are closer together and buds are smaller and looser with fewer trichomes and less scent. According to Jones, cannabis growers can lose a majority of their harvest to HLVd.

What do we know about HLVd so far?

It is likely transmitted through clones, and is spread from plant to plant through manicuring utensils and handling, as the ‘blood’ of the plant gets on scissors or gloves.  Established sterilization practices using isopropyl alcohol do not kill the viroid. In fact, alcohol seems to amplify HLVd, making plants more susceptible to infection, Jones says. Because it lies latent, it is often spread unknowingly within and between cannabis grows. 

Healthy cannabis plants compared to those "dudding out" with the hop latent viroid

Healthy cannabis plants compared to those “dudding out” with the hop latent viroid. Photo by Dark Heart Nursery.

When did it crop up?

Jones says he first started hearing about duds in the ’90s, at the same time he was working from Oakland, Calif. to normalize clones and distribute them cheaply to medicinal patients far and wide. Prior to Jeff’s work, cannabis was grown from seeds, and seeds do not seem to transmit HLVd.

Why is it so insidious?

Cannabis prohibition prevented the academic study of what was causing the duds, but that is changing. Now that more and more states are legalizing cannabis and cultivation is going commercial, companies are investing in a solution. Yet, commercialization also means the problem seems to be spreading at an even faster pace as growers bring new cultivars into their farms, facilities, and greenhouses and share cultivars with others.

How can we stop it?

That’s the multimillion-dollar question. Here’s what’s being done so far:

  • Testing: Growers are investing in rapid testing that screens clones for HLVd before mixing them with other plants. 
  • Propagation Techniques: Using tissue cultures to grow cannabis from a single cell that can potentially be stripped of HLVd are promising. 
  • Sterilization: Bleach and hypochlorous acid also seem to work. For large grows, expensive water electrolysis machinery can be used to spray plants twice a week. Small operators can buy relatively inexpensive jugs of these disinfection mixes and literally spray, soak, agitate and brush plants individually to prevent spread. 

Still, Jones cautions, none of these methods are failproof. Plants can test clean, then wind up with HLVd down the road. “It can be latent in the plant, then you can stress the plant out and it comes back again,” Jones says. “Just because you’re free of the virus doesn’t mean you’re free of the viral load.”

Is HLVd returning due to local contamination? Did it come hidden in the plant despite clean test results? These are questions researchers are struggling to answer — and fast. Oaksterdam’s experts are staying on the forefront of this issue, communicating with the cannabis community and freely sharing information as it becomes available. 

For more information on HLVd, Jones recommends this white paper by Dark Heart Nursery. Join Oaksterdam’s Plant Talk on Clubhouse, Fridays at 4:20 PT, to have your questions about the hop latent virus and other issues answered by OU experts live. For a deep dive into cannabis cultivation, sign up for our Live Semester or Self-Paced Horticulture Courses here.