Hop latent viroid is a recently identified plant-specific pathogenic RNA. With the ability to infect cannabis, some in the industry think this may be the major cause of previously unexplained “dud” plants during clone propagation.

Here we’ll explore the nature of this infectious plant disease, its known symptoms, and how to prevent transmission in your grow operation.

Hop Latent Viroid: A Novel RNA Virus

The story of Hop Latent Viroid or HLVd first started in California in 2017 when cannabis grower Graham Farrar of Glass House Farms noticed strange symptoms and reduced yields in a subset of his plants.

Baffled as to what was causing these plants to become so sickly, Farrar reached out to researchers at the University of California Davis, known for its strong plant science program and ties to the agricultural industry.

This eventually led to a successful research collaboration with Phylos Bioscience, an Oregon-based cannabis genetic testing company. Researchers at Phylos sequenced samples of Farrar’s sick plants and identified a novel RNA virus.

The results were published in 2019 in the journal Plant Disease and independently verified by another research group affiliated with Dark Heart Nursery in Oakland, California. They named the novel RNA virus Hop Latent Viroid. Unlike a traditional virus which relies on DNA to infect its host, RNA viruses, like the SARS-CoV-2 virus responsible for COVID-19, use RNA.

HLVd Symptoms in Cannabis

HLVd causes noticeable symptoms in cannabis plants including:

  • Growth stunting
  • Brittle stems
  • Reduced trichomes and oil production
  • Reduced flower mass
  • Malformation and/or chlorosis of the leaves

Researchers now think HLVd is likely widespread among cannabis growers across the country. However, due to its very recent identification, many growers may not be aware of its existence or familiar with its symptoms.

Another confounding factor in controlling the spread of HLVd is its tendency to lay dormant. As its name suggests, the virus can remain latent or inactive within infected plants, showing no discernible symptoms.

HLVd’s latency allows it to quietly spread until a secondary stress from heat, nutrition, or a pest allows the virus to activate and take hold. This makes a new outbreak difficult to detect without testing. Clones propagated from a single infected mother plant are especially vulnerable and researchers believe HLVd may be the cause of seemingly random episodes of clonal dudding.

Cuttings taken from an infected plant for clonal propagation tend to have lower rates of rooting success. But if the virus remains latent and the clones make it past rooting, an environmental stressor during the growth phase could cause between 10-30% of your plants to succumb to HLVd.

Preventative Measures

There’s still much to be learned when it comes to the disease transmission and progression of HLVd in cannabis. However, it appears that the virus is mainly spread through mechanical means or fomite transmission which involves contact of a susceptible host with a contaminated object.

The good news is that you can prevent this type of transmission by using best practices for cleanliness. Sanitize all of your tools prior to propagation. A diluted bleach solution may be more effective than alcohol in killing virus particles. Also consider quarantining and carefully inspecting any new varieties you receive before adding them to the rest of your plant stock.