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What is Hop Latent Viroid?

June 16, 2022

Hop Latent Viroid on Cannabis plant

By Shaun Stice, PhD

Cannabis growers today face a pathogen threat to their crop known as Hop Latent Viroid. What is Hop Latent Viroid and what do you need to know about the viroid causing $4 Billion in Cannabis crop losses annually?

Before answering this question, we might wonder what is a virus? And how does a virus contrast with a viroid? A virus is an infectious agent essentially nucleic acids (RNA or DNA) that are packaged up in either a protein coat or a cellular membrane. Most everyone is familiar with a virus at this point considering the COVID-19 pandemic. The COVID-19 virus (SARS-CoV-2) has a single-stranded RNA genome that is encapsulated with a lipid membrane. Plants just like humans and animals can be infected with various viruses, but viroids are a unique biological affliction that can impact flowering plants.

Viroids are a single stranded piece of RNA that form a circle. They are extremely small. For perspective, the average plant virus in the potyvirus family has about 10,000 bp of genetic code (RNA) that encodes for ten proteins. The average plant virus genome encodes proteins that enable the virus to enter, replicate, and spread to new host cells. 

Hop latent viroid (HLVd) is only about 250 bp in size and does not encode any proteins. Instead of encoding proteins to replicate, HLVd has a specific signal in its RNA code that hijacks the host cell RNA polymerase II to replicate its sequence, the RNA sequence of HLVd has enzymatic activity by itself and can self-cleave into new naked circular sequences as it is replicated in a mechanism called the “rolling circle mechanism.”

A unique threat to Cannabis

With such a small genome it has been a big question in plant pathology as to how these tiny bits of RNA can impact the plant in such a profound way. The current scientific consensus is that the presence of the viroid triggers the plant’s immune system to attack itself. Specifically, the presence of the viroid triggers the RNA-silencing pathway, activation of this pathway is normally important for plant defenses against viruses, however in some cultivars or hosts the plant may silence its own vital cellular functions thus causing the typical symptoms associated with viroid infection.

The HLVd viroid was originally characterized in Hops (Humulus lupulus) which are closely related to Cannabis (Cannabis sativa) as they are both in the family Cannabaceae. Thus, HLVd can infect Cannabis plants quite readily. HLVd is spread mechanically through dirty cuttings, close plantings, or clonally by propagating an infected mother plant. The term latent is included in the name of HLVd because symptoms are not readily apparent when a plant is young, but symptoms such as stunting, malformed chlorotic leaves, reduced flowering and yields may present as the plants reach maturity. 

What’s at stake? Dollar and cents, of course

Virus and viroid infection can cause economic losses in plants, and it has been recently shown that HVLd infected hops and Cannabis plants may have reduced vigor, and reduced yield when it comes to flower production and the creation of important metabolites such as THC and CBD in cannabis and economically important bitter acid compounds in hops. This translates into more than $4 billion in annual losses for US growers who are forecast to produce close to eight million pounds of legal cannabis in 2022.

How to combat the threat

Cannabis lab testing propagative material is the most effective way to prevent the spread of HLVd. Once identified, the first step to control any virus or viroid is to start with clean plant materials. There are a variety of molecular tests on the market today that enable growers to test for the presence of HLVd in their mother plants or starting materials. The presence of HLVd in your plant does not necessarily mean you should destroy the plant. 

There are complex interactions that lead to reduced vigor and or yield loss with plant viroids. The severity of symptoms or yield loss is dependent upon the strain of the viroid, the genetic background of the host (cultivar), and environmental factors (indoor vs. outdoor, disease pressure, nutrient deficiency etc.). If you suspect that your plants have HLVd and are seeing a large reduction in yield in a specific cultivar it may be important to screen your plants as HLVd may be a contributing cause of decline. 

If you have some cultivars that are infected with HLVd but do not seem to exhibit a reduced yield, you could try to keep these plants separate from other cultivars to prevent the spread of HLVd to cultivars that might have more severe response to the viroid. Prioritize HLVd testing for your starting material, then take necessary action to neutralize the spread.

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