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An insidious blight that is ravaging West Coast marijuana crops has landed in Massachusetts

The highly transmissible “hop latent viroid” drastically reduces cannabis yields and quality but has no known effect on human health

A worker analyzed samples of marijuana in California, where a plant disease is affecting marijuana yields.Rachel Weill/Dark Heart Industries

An insidious plant disease wreaking havoc on West Coast marijuana crops has landed in Massachusetts, worrying local growers who say it could significantly reduce harvests at a time when cannabis operators are already confronting significant business headwinds.

The so-called “hop latent viroid” has no known effect on human health, and state regulators don’t currently require growers to test for its presence. But plants infected by the highly transmissible and difficult-to-detect disease typically produce much smaller quantities of lower-quality marijuana than projected, meaning companies are unlikely to make back the money spent on the crops.

The industry researchers who first identified the viroid as the cause of mounting crop losses found that 90 percent of cultivation sites in California from 2018 to 2021 had at least some infected plants, and estimate the blight may have already cost the US cannabis industry up to $4 billion in lost yields.

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Now, state regulators, cannabis testing labs, and workers and executives at commercial cultivation facilities in Massachusetts say the disease is becoming increasingly common here, too, sending some cultivators scrambling to implement rigorous and pricy controls to keep the viroid at bay.

“A single room [of marijuana plants] can be worth hundreds of thousands of dollars,” said Gabriel Lodoño, the chief executive of Chelsea-based marijuana company Harbor House Collective, which recently destroyed numerous plants after noticing unusual patterns in the leaves caused by the disease. “If you lose 25 or 30 percent of that, it really hurts.”

The viroid first reared its head in the commercial cannabis sector around 2012, when growers in California began noticing a strange series of symptoms afflicting their crops: Unusually brittle branches that grew sideways instead of vertically, shrunken leaves with subtle discolorations, and significant reductions in the quantity and potency of pot yielded by the plants.

Seven years later, industry researchers definitively identified the cause as the hop latent viroid, a primitive plant disease first described in a 1988 study of the hop plants used in beer production. But by then, it was too late. Cultivation experts and cannabis testing labs say the viroid today is practically ubiquitous, fueled by a huge increase in the exchange of cannabis clones and seeds across state lines as more licensed growing facilities open and seek out desirable strains.

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“This is no longer just a California problem; this is an international problem,” said Jeremy Warren, a plant pathologist for Dark Heart Nurseries in Oakland who in 2019 was the first to show that the viroid was the cause of the symptoms noticed by cultivators. “It’s in Canada for sure. We’ve gotten calls from Spain and Portugal. It’s everywhere. Almost every facility we’ve ever tested has some amount of it.”

Much about the viroid remains mysterious, including which plant it initially evolved to target. Unlike many other cannabis diseases and pests, the hop latent viroid is difficult to identify through visual inspection on juvenile plants, and some never show visible symptoms. Even testing at an offsite laboratory frequently gives false negatives, suggesting it is not uniformly present throughout individual plants.

The viroid is also highly transmissible through contact, readily leaping from plant to plant on the shears that workers use to prune leaves. It can even spread through the seeds of infected plants, though at a lower rate. And worse, the disease is untreatable. Cultivators with an infected crop essentially have only two choices: Cull the affected plants or produce inferior pot in smaller quantities.

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That leaves prevention as the best option. But keeping out the viroid costs both money and time, requiring cultivators to constantly and meticulously sanitize pruners and other equipment while also paying to repeatedly test plants.

Some cultivators are calling on state marijuana agencies to mandate that breeders and cultivation facilities test all new strains before launching full-scale production, saying such a rule would prevent crop losses.

Greg “Chemdog” Krzanowski, a legendary underground grower who now manages cultivation operations at Massachusetts marijuana firm Canna Provisions, said he has started to test for hop latent viroid in the popular clones the company sells to home-growers through its dispensaries. Requiring the same for breeders and seed providers, he added, should be a no-brainer.

“These giant nurseries in California that are collecting a huge number of clones from different people and then selling them to companies all around the country — that’s a viroid contamination festival,” Krzanowski said. “It’s one little regulation that would do a lot for the industry, even if it does slow things down a little bit.”

Implementing such a rule could be difficult, however. Because interstate commerce in pot is nominally banned, most states allow licensed cannabis growers a brief window to discreetly acquire new seeds, clippings, and tissue samples of different strains from anywhere in the world before later declaring them to regulators and using them to propagate production plants. It’s a system that relies on a principle of “immaculate conception,” or perhaps “don’t ask, don’t tell.”

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Similarly, the national prohibition on cannabis has sidelined the federal government, which for other crops helps prevent blights by working with universities and other institutions to breed resistant plants.

Some producers are investigating novel ways of exchanging genetics and starting new crops that are less likely to transmit the disease. But the most promising technique — cloning plants from tissue cultures — typically requires the use of hormones that are banned under Massachusetts regulations. Lodoño called on the state Cannabis Control Commission to loosen that restriction.

“As the market grows and more states legalize and more companies try to push into different states and share their genetics, the risk of introducing the viroid every time you get a new strain is just going to increase,” he said. “Without propagating from culture, there’s no way to know, but the regulations right now make that really tricky.”

For now, the problem of the hop latent viroid has largely fallen to growers themselves.


Dan Adams can be reached at daniel.adams@globe.com. Follow him on Twitter @Dan_Adams86.