A plant pandemic?

Hop latent viroid isn’t new, but it’s becoming a larger problem as the cannabis industry grows

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It’s an RNA-based viroid that is extremely contagious—to avoid spreading between individuals it requires careful social distancing and diligent disinfection. It’s wreaking havoc in places like California, Michigan, and Colorado. Some infected individuals won’t show any symptoms whatsoever, while others will wither fairly rapidly after onset. Worst of all though, is the fact that symptoms often develop long after initial infection. 

And while there isn’t a cure for it (yet), companies like PSI Labs out of Michigan and Dark Heart Nursery out of California highly recommend regular PCR testing to gauge infection rates. Otherwise, growers could be losing 20 to 30 percent of their crop every grow cycle. 

The disease is called latent hop viroid (HpLVd) and it’s gouging grow operations across the country. In California alone, where Dark Heart Nursery tested over 200,000 samples of cannabis tissue from more than 100 licensed cannabis vendors, over 90 percent of their grow facilities tested positive for HpLVd. The company estimated that could constitute $4 billion in crop losses. 

According to Dr. Lev Spivak-Birndorf, a co-founder of PSI Labs, one of the only cannabis PCR testing companies in the nation, the pathogen is visually observable—you can see it, he says. The flower of HpLVd-positive plants is noticeably lackluster: the leaves are swollen; the stems get weak and the quality of the bud is dramatically lower. The trichomes don’t develop, and there is actually less cannabinoid and terpene content in the bud. 

And the disease doesn’t affect all plants equally. Some will outwardly exhibit symptoms, while others will exhibit none—carrying the disease asymptomatically. 

“The good news is it’s not a safety issue in the sense that smoking virally infected plants is not going to harm humans,” says Spivak-Birndorf. “It’s more about the fact this is devastating the quality of crops and that’s becoming very costly to growers.”

Hop latent viroid is not a new disease, Spivak-Birndorf explains. As the name implies, it was first discovered in hops (a close cousin to the cannabis plant). It caused problems for commercial hop growers long before it started wreaking havoc on cannabis farmers. But now that cannabis is being grown commercially, it’s sneaking up to take a commercial-sized bite out of those business profits as well. 

And HpLVd is sneaky. To Spivak-Birndorf, the most pernicious feature of this disease is the “latent” aspect of it.

“Plants don’t really start showing [symptoms] until you’re pretty far into the flowering stage,” he says. “It’s not something you can identify early on, in most cases.” 

And if a grower identifies it at all, it’s probably too late. The viroid is so contagious, if two plants brush up against each other, it could be spread between them. Or, if someone at the grow is using the same pair of shears to trim a row of plants, they could potentially be spreading it manually throughout the grow. 

Spivak-Birndorf thinks this is one of the primary drivers in the spread of the disease. 

“Hops are grown in rows, and a lot of times [HpLVd] will be spread down the rows, correlating with people nipping with the same shears and going down the line,” he explains. “It really does the same thing to cannabis. In a way, those two things are very parallel.”

Cloning (which is the main method of propagation in commercial cannabis grows) presents problems of its own, as HpLVd can be passed on genetically. And because plants can carry the disease asymptomatically, a healthy-looking mother plant could be riddled with HpLVd. And mysteriously, says Spivak-Birndorf, its clones might not have the same genetic resistance.

“We’re seeing cuttings from the same mother, in the exact same room—the same clones—and some [of the plants] will be fine. Others will somehow have a trigger that causes that virus to take over.”

That won’t happen until those clones reach maturity and start flowering, due to the latent nature of this pathogen. There’s no way to know if they’re infected with HpLVd until then. Except regular PCR testing, which Black Heart recommends doing on every mother plant every month and a half. In their assessment, growers who completed PCR tests as advised and who immediately destroyed HpLVd-positive plants upon identification reduced infection rates from 40 percent to just 1 percent. 

Spivak-Birndorf says that PSI Labs’ results back that up. They’ve seen growers who identify an outbreak early and who take fast action to address it significantly mitigate their losses. But he’s also careful to temper expectations about PCR testing. 

“I don’t like to kind of oversell the information [PCR testing] provides, because in some ways, if you have [HpLVd], some things are going to go bad and some aren’t,” Spivak-Birndorf says. At that point, he continues, “Growers have to make hard decisions about what they want to keep, what they want to get rid of, and how to approach a program of breaking down, sanitizing and rebuilding a room to make sure that you can kind of keep [HpLVd] out.”

There’s still a lot left to learn about HpLVd and about cannabis pathogens at large, according to Spivak-Birndorf. Moving forward he believes that understanding this disease and others like it, how they spread, and how to fight them, is going to become a massive area of interest in the cannabis industry. 

“The next cannabis pandemic could be around the corner,” Spivak-Birndorf says “Really, we just don’t know.”