Weed disease hop latent viroid is coming for your plants

Across North America, something called Hop Latent Viroid (HLV) is wreaking havoc. This virus-like infection can make plants sickly and destroy harvests. It’s highly contagious. Studies have estimated that perhaps 40% of cannabis flower sold legally in Canada carries HLV. As much as 90% of cannabis in California might be infected, costing billions of dollars in lost yields. 
What exactly is Hop Latent Viroid (HLV), how does it work, and what can growers do to protect their precious Cannabis plants?

Hop Latent Viroid: What are viroids & How are they different than viruses?

Viruses are tiny infectious agents. They can infect animals, plants, or single-celled organisms. They are much smaller than even a bacteria cell, consisting of a small piece of genetic material (DNA or RNA) protected by a protein shell. These protective shells help preserve the genetic material of the virus and contain various proteins enabling them to infect specific host cells. 

Viroids are similar to viruses, but different in key ways. Viroids do not have a protective protein shell. Instead, they are small circular strands of RNA. They seem to specialize in infecting flowering plants. In other words, they are tiny, “naked” pieces of genetic material that infect certain plant species, causing disease. When they infect valuable crops grown by humans, such as Cannabis, this can have a devastating economic impact. 

The “goal” of viruses and viroids is the same: replication. They cannot reproduce on their own. They must come into direct contact with the right host cell, smuggle their genetic material inside, and hijack the cell’s replication machinery. Eventually, the host cell fills up with viral particles and bursts open. When you get sick with a viral infection–such as COVID or the common cold–it’s because your immune system is responding to a large number of these viral particles circulating throughout the body.

Hop latent viroid: What does it do to cannabis plants?

Hop Latent Viroid is a viroid that infects hop plants, which are used to brew beer. Cannabis is a relative of hops. In recent years, HLV jumped from hops to Cannabis. Infected plants show various defects, ranging from stunted growth and reduced foliage to uneven trichome coverage and decreased cannabinoid production–symptoms of what’s been called “duds disease.” This is a huge problem for cannabis growers, whose livelihoods depend on reliably growing healthy, cannabinoid-rich plants with bountiful harvests.

Cannabis plants infected with HLV show obvious outward defects: smaller overall sizes, reduced root development, and discoloration. Here are some pictures. They have smaller flowers (the part of the plant meant for human consumption), and can produce up to 50% fewer cannabinoids, like THC.

How does hop latent viroid (HLV) spread in cannabis

Similar to viruses, viroids like HLV need to come into direct contact with their hosts to infect them. Cannabis plants can catch HLV when they come into physical contact with infected plants. Although HLV doesn’t infect humans, we can spread it between plants through contact with body surfaces, tools, or equipment. Contaminated water supplies are also a major source of infection, as HLV tends to concentrate in the roots. 

All of these potential points of infection enable the rapid spread of HLV, as Cannabis plants are often grown in high density, require human contact at multiple points of the production process, and can be connected to common water supplies (e.g. in hydroponic systems).

Hop latent viroid can also spread from mother plants to offspring, both through clones generated by taking cuttings and through seeds. All offspring can potentially carry HLV if their parent is infected. This makes it essential to identify infected plants, even if there are no obvious outward signs of infection or you’re working with tissue culture systems with physically isolated samples.

Because HLV is so contagious, it has already spread widely and caused mass losses for Cannabis growers. It is likely to continue spreading. Growers must be prepared.

How can growers protect against hop latent viroid (HLV)?

Whether or not growers are already battling HLV, they need to have processes in place to test and remove infected plants. It is obviously important to learn how to visually identify potentially infected plants, but it’s always possible to miss subtle signs.

As far as I can tell, the only reliable way to be sure whether plants are infected is to conduct genetic testing, similar to what would be done to detect something like COVID infection in yourself. A sample must be taken from a potentially infected individual and subjected to a laboratory test capable of detecting the presence of genetic material from a particular pathogen. 

For Cannabis growers, this means either developing in-house capabilities and purchasing test kits, or sending samples out for testing elsewhere. Any plants known or suspected to be infected with HLV need to be immediately removed to prevent the spread of infection. Plants in close proximity, even if they show no signs of infection, should be quarantined or monitored closely.

Dr. Zamir Punja’s research team has conducted studies looking at how well HLV infections can be managed using a test-and-remove approach. They were able to reduce the percentage of infected plants from 35% to 7% over a period of seven months. In other words, handling a HLV outbreak is likely to be costly in terms of time, labor, and money. For commercial Cannabis growers, it is advisable to have a robust detection process in place, before isolated infections turn into full-blown outbreaks. Being diligent could mean the difference from a few infected plants vs. the loss of an entire harvest. 

Other preventative measures should also be taken. HLV is surprisingly stable on surfaces, with the ability to survive for days or even weeks on equipment or plant material. This viroid is also apparently capable of withstanding high heat, UV radiation, and disinfectants to some extent. For these reasons, growers need to be proactive and conscientious about hygiene. Are tools and equipment being fully sterilized between uses? Are supplies and staff traveling between rooms with different plants? How often are disposable items being reused and thrown out?

Any large-scale growers with high-density grow operations should be prepared, especially if their plants share common water supplies, nutrient sources, and soil. Given the rapid spread of HLV, Cannabis cultivators everywhere should be prepared. As I’m sure we all know by now, viral outbreaks are difficult to manage and can be highly disruptive. Growing Cannabis is hard. It’s a science and an art form. In an already competitive market with slim profit margins and the inability to deduct normal business expenses due to the schedule I legal status of marijuana, every harvest counts.

Be prepared.