Some cannabis cultivars resistant to hop latent viroid symptoms, research shows

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Image of a cannabis plant infected with hop latent viroid next to a healthy plant

The cannabis plant on the left is infected with hop latent viroid; the plant on the right is healthy. (Photo courtesy of Zamir Punja)

Gelato, anyone?

Hop latent viroid, already infecting most cannabis grows across the United States and Europe, is coming under greater scrutiny as researchers scramble for options to control the spread of the parasite, including making the plant resistant to the viroid.

Some cannabis cultivars, such as Gelato 33, are showing a natural level of resistance, although it is unclear what variables contribute to their increased defenses.

Viroids are remnants of ancient RNA that preceded DNA and proteins during evolution.

But hop latent viroid (HLVd) is the enemy of cannabis cultivators everywhere.

First identified in 1987, HLVd plagues cannabis growers around the world by mutating plants, stunting their growth and drastically lowering THC levels.

According to a study published in the journal Viruses, HLVd causes $4 billion in losses to cannabis growers each year.

How HLVd is spread

Hop latent viroid is highly contagious and transmitted mechanically from utensils used to harvest and process plant material, through shared water or insects moving between plants.

It also can exist on the seed coat or within the seed itself – and it can live there for a while.

HLVd has been detected on cannabis seeds stored for more than two years.

Studying pest resistance

Researchers still are scratching their heads about how to stop hop latent viroid and, perhaps, make the cannabis plant resistant to it.

“At the moment, we have no data on what could make a cannabis strain more resistant to hop latent viroid,” Zamir Punja, professor of biological sciences at Simon Fraser University in Vancouver, British Columbia, told MJBizDaily.

Researchers worry that widespread viruses affecting other crops, such as alfalfa, soybeans and tobacco, could make their way to cannabis at some point.

Worse yet, according to a study published in the International Journal of Molecular Science, there is some evidence for the potential evolution of new and genetically diverse virus/viroid strains that can infect and become established in cannabis crops.

Prevention in practice

Rob Baldwin, vice president of cultivation and greenhouse operations for Pure Sunfarms in Vancouver, said if the viroid infects a mother plant, it can spread to the grow through clones.

to ensure its clones are clean before they enter the greenhouse.

Since beginning operations in 2018, Pure Sunfarms has used various testing regimes to screen its plants for different types of disease and infection, including testing plant material for the hop viroid.

When HLVd symptoms first were detected, Pure Sunfarms quickly ramped up its testing procedures to identify whether the pest had permeated the grow.

The company routinely tests all mother plants to avoid transmission of the pathogen.

‘Showing no symptoms’

“Some plants’ certain genetics for one reason or another are going to be less susceptible to infection,” Baldwin said.

“That probably comes from the hereditism that was carried on from breeding. If it had more landrace in it, it might be more resistant,” he said, referring to strains that evolved to thrive in their native environments.

Because of the latent nature of the viroid, it is difficult to say which strains are less susceptible to it, Baldwin said.

“Some strains showing no symptoms of the viroid may present later in their life cycle,” he added.

Identifying HLVd susceptibility

Cannabis plants do not have a natural resistance to the viroid, yet cultivators are noticing that some cultivars are less susceptible to HLVd infection than others.

That sort of asymptomatic action to HLVd happens in different tomato cultivars as well.

Researchers hope that studying certain cannabis cultivars and other plants will help identify the pinnacle of true plant resistance.

A recent experiment on 12 different varieties of cannabis – half of which were purposely infected with HLVd – revealed other interesting details about the viroid, hinting at how to make the plant resistant to it.

The plants were tested every three to four weeks until harvested, with researchers recording metrics about growth and yields in different varieties to help understand how HLVd symptoms correlate to different cultivars and the amount of viroid present in the plant.

Best time to test

According to Tassa Saldi, chief science officer at Colorado-based Tumi Genomics and head of the experiment, one of the study’s findings concerned the best time to test for HLVd.

Testing when the plant was coming out of propagation, as Baldwin does at Pure Sunfarms, “was the most predictive of whether or not that plant was going to grow up to have severe symptoms,” Saldi said.

“If the plant was negative at that point and became positive – or it was undetectable at that time point (and) became detectable later on in flower, we tend to not really see a negative impact on yield.

“If people are looking for a specific time point, the experiment was telling us that (the immediately post-propagation) time point is the most predictive.”

Saldi continued, noting, “Then, we also discovered (the Illemonati cannabis strain) that had a ton of viroid, but it just didn’t care.

“It was at a really high level of viroid, and the plant didn’t have any symptoms even though it was super infected.

“That was partly because it was difficult to infect them. So, the viroid load stayed low at the beginning.”

Gelato 33, Motorhead and Oreoz

Cultivars that demonstrated low or essentially nonexistent levels of viroid after three weeks included Gelato 33, Motorhead and Oreoz.

Chilled Cherries, Purple Milk and Wedding Pie, meanwhile, showed very high levels of HLF infection after the same period.

“The idea would be to find enough resistant, sensitive and tolerant varieties that we can look for the gene or set of genes in the DNA that’s responsible for that resistance or tolerance,” Saldi said.

“Then, theoretically, you could create your favorite strain but alter that one gene so that it is now resistant or tolerant to the viroid.

“First, we have to figure out what that gene or set of genes is.”

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Cannabis gene editing

Removing a gene or changing a DNA sequence would have to be done using Crispr-Cas9 gene-editing technology.

But the work to identify the gene responsible for HLVd sensitivity hasn’t yet been started, Saldi said.

Crispr-based virus resistance in plants has been achieved by gene targeting and cleaving the viral genome – or altering the plant genome to enhance plant innate immunity – according to a study published in Current Genomics.

Researchers point to an understanding of viroid-host interactions, host susceptibility, host response to viroid invasion and non-host resistance as possible guidance for the design of viroid-resistant plants.

Stopping the spread of HLVd

For now, keeping the viroid in check involves intense testing, sanitation of the facility and eradication of infected plants.

Pure Sunfarms used that methodology to reduce the incidence of HLVd in its operation from around 25% to 1% in about eight months.

But there’s more to do, according to Baldwin.

“Cannabis breeding is still a little behind other crops,” he said.

“People are working on breeding other disease resistance into, for example, powdery mildew. I think that’s the way to go.

“We see that some (cultivars) get the viroid easier than others, so there must be something there.”

If research collaborators working on HLVd shared data, Saldi said, “we could get (viroid resistance) accomplished in, I’m guessing, five years.”

“The science is super unpredictable,” she said.

“And some of it will probably come down to luck in finding those genes and then determining whether or not we can alter them.”