How the Hop Latent Viroid (HLVd) Threatens Licensed Producers

The hop latent viroid disease, often abbreviated as HLVd (sometimes HpLVd), is an infectious pathogen of hops, and its related species, cannabis. It was first identified on hops in 2017 in California but has since been found throughout the U.S. and Canada. Though it was not named as such until 2017, it was probably already in place years before, and its damage had been ascribed to a PCIA, or putative cannabis infectious agent. An estimated 40 percent of licensed growers in Canada alone have had, or are currently experiencing, HLVd on their cannabis crops. It affects growers of all sizes and scales.

It is responsible for what has been called “dudding disease,” or sometimes just “dudding.” Though most often transmitted by clone propagation, it can be passed down through the seed of an infected plant. It is most often unknowingly spread during the propagation process or through other mechanical means. The viroid is spread between and among plants often through contaminated pruning tools or even workers’ gloves. At this time, it is not thought to be spread by insects like aphids that may travel from plant to plant.

Note that this pathogen is not a “virus.” Viroids are the smallest known pathogens of crops. They differ from viruses in that they are smaller and have no protein coating encapsulating them. Like some viruses though, they are circular, single-stranded RNA molecules. The discovery of the first viroid was made in 1971. Many more have been discovered since then and there are now at least 30 identified viroids.

There are many reasons growers are taking note of this disease or should be paying attention to it. It is extremely difficult to detect. Not all plants infected by HLVd show any symptoms at all. In the case of its namesake host, hops, the disease is always asymptomatic. Growers have a lot to be concerned about, and rightly so.

Why Producers Should be Concerned

Hop latent viroid is a disease that seemingly comes out of nowhere. Growers can get all the way into the flowering stage, even the pre-harvest stage, and not know that their plants have been infected. Not all cannabis plants show symptoms when they have the disease. This means it can be very difficult to limit the spread of this viroid.

The latent nature of this disease means it can spread without notice from plant to plant undetected throughout any phase of growth. Most growers who have this disease are not aware of it until it’s too late. A single infected clone or plant can infect an entire crop.
What is the chance of any particular crop getting devastated by this disease? The best guess of researchers is that HLVd is widespread, and knowledge of it amongst growers is lacking, meaning it will continue to spread. Between 30 to 40 percent (some estimates are up to 50 percent) of a grower’s crop of cannabis is estimated to be lost by each producer as HLVd continues to run wild throughout North America.

We don’t know what we don’t know. Hop latent viroid disease is relatively new. The prohibition on growing cannabis in the U.S. and Canada for so long meant that government and academia were not studying it and finding treatments for its more common ailments. Common problems like powdery mildew or aphids have solutions that have been studied and researched for decades. Hop latent viroid research has no such head start.

What we do know is that affected cannabis plants have lower cannabinoids and terpenes. THC and CBD levels are all lowered by HLVd. We also know infected crops need to be destroyed and any facility or tool that may have come in contact with an infected plant needs to be thoroughly sanitized.

Symptoms of HLVd

As mentioned, the symptoms of the HLVd are difficult to discern as they present like so many other diseases, don’t show up until later in the flowering phase, or don’t show up at all. Cannabis plants with HLVd start their life out normally and may appear totally healthy. Once they approach or enter the flowering stage, however, infected plants that are symptomatic begin to show signs of the disease.

Common symptoms include stunted growth with brittle stems. They may also express abnormal branching, with smaller than average and spiky leaves. Chlorosis can set in, yellowing the leaves. The nodes on the branches are closer than they should be, and any flower buds are noticeably smaller than they would be on a healthy plant. The buds are looser as well. Possibly the most significant effect of HLVd is that trichome numbers are greatly reduced, as well as oil production. The scent of affected buds is noticeably less intense as well.

Unfortunately, HLVd cannot be treated like other plant diseases. It can only be prevented. The good news is there are several steps that can be taken to prevent it getting into your growroom or infecting the crops in your fields.

Prevention and Treatment of HLVd

There is no such thing as “too” clean when talking about preventing HLVd. While we are still very far from knowing everything about this disease, there are some practices we know that will keep your crops safe from it. Those practices that keep your crops and plants safe from other plant viruses and viroids will keep your plants safe now. It starts with genetics. Make sure to buy seed, clones, or plants from reputable sources that test their strains for diseases including HLVd. This won’t prevent plants from contracting the disease if exposed to it, but it should ensure you are starting out with a clean slate. Purchasing genetics from tissue culture is a great way to ensure clean stock (more on that below).

Once you have your plant material, it is imperative to keep every aspect of the process as clean as possible. HLVd can be, and often is, spread by transmission between plants via infected tools, machines, even being carried on people’s clothes. Keep a separate set of tools to be used for each individual growroom, field, etc., and keep those tools clean! It is believed that alcohol does not kill the viroid — it has in fact been shown to make the transmission of HLVd even easier. Instead, use bleach or hypochlorous acid.
Some other practices to help avoid spreading hops latent viroid disease:

• Quarantine any suspected plant and any plant it was touching.
• Quarantine new varieties of cannabis received before adding to existing stock.
• Spread out the plants in your growroom so they cannot touch one another. Leave plenty of space between field-grown plants.
• Get a diagnostic test done as soon as you suspect one of your plants may have HLVd.

Future Prospects of HLVd in Cannabis

There has been some promising progress dealing with the avoidance of HLVd in cannabis. New tests are being developed that can quickly screen clones for HLVd before they have left a growing facility and can infect other plants. Advances in propagation techniques may also be part of the future of avoiding HLVd.

As little as one single cannabis cell using tissue culture technology may be able to grow a cannabis plant that has been stripped of HLVd. Growers who have a proprietary strain, or one they want to keep producing won’t have to give them up if they have the disease. Labs are showing the ability using tissue culture to get rid of HLVd, leaving the strain like new. New, HLVd disease-free cannabis plants can be created from infected tissue using cutting-edge techniques.

Once HLVd has been detected and positively identified in a plant (through lab confirmation), then the next step is to get a sample of a root tip or apical meristem from the infected plant. Several samples should be taken before the crop is destroyed, and the infected facility has been scrubbed.

The process to completely eradicate the viral load from the strain can take upwards of a year. At the end of the process, though, a grower will have growing stock — that he or she can be certain of — that does not have any trace of HLVd.

Until these types of methods become routine, there is still ongoing research into resistance to HLVd. Tests of different cannabis strains will continue to be exposed to HLVd in testing facilities to determine which strains may have natural resistance. It is entirely possible that new cultivars will be developed as a result of this testing, blending strains with resistant traits to create new ones that are fully resistant to the disease.

Until then, starting with certified clean material and keeping your operation clean and sterile is any grower’s best hope for keeping HLVd at bay.