Pot farms in California are under attack. According to a new report, more than 90 percent of the state’s cannabis farms are infected with a “severe” pathogen.

After growing invisibly for months, an infectious pathogen called hop-latent viroid, or HLVd, is now ravishing cannabis plants and spoiling crops, causing billions of dollars in damages to the national weed economy, San Francisco Gate reported.

HLVd shrivels pot plants by as much as 30 percent. It also destroys the amount of Tetrahydrocannabinol (THC), pot’s most common active compound, that a plant produces, greatly reducing the value of affected plants. 

A study confirmed the viroid’s presence in samples from a Santa Barbara pot farm. It’s now infected at least 90 percent of California’s cannabis grows, according to a 2021 estimate. It’s spreading globally, and a recent scientific paper declared the pathogen was the “biggest concern for cannabis” growers worldwide.

It’s most likely, said researchers, that HLVd has been spreading in cannabis farms for more than a decade, but growers didn’t know what was harming their harvest. HLVd can destroy the retail value of a crop.

Cannabis plants that have been infected with HLVd can have smaller leaves, stunted growth, malformations, yellowed leaves, and a reduced flower mass and trichomes. The buds of infected plants are usually smaller as well, according to online publication Analytical Cannabis.

It was in the 1980s that HLVd was first identified in hop plants, a close relative of cannabis. In 2019, HLVd was first identified in cannabis when scientists studied samples from a Santa Barbara, California, pot farm. The viroid has spread to pot farms across the world, from Massachusetts to Europe, San Francisco Gate reported.

While it is highly transmissible from plant to plant, the hop latent viroid has no known effect on human health, according to the study’s researchers and life sciences company Medicinal Genomic. But it is deadly to the legalized cannabis industry. It may have already cost the U.S. cannabis industry up to $4 billion in lost yields.

“This is no longer just a California problem; this is an international problem,” Jeremy Warren, a plant pathologist for Dark Heart Nurseries in Oakland, told The Boston Globe. In 2019 Warren 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.”

It may take some time for growers to figure out how to combat the viroid as it remains mysterious, including which plant it initially evolved to target. It is also very difficult to identify through visual inspection on juvenile plants, and some plants never show visible symptoms. It is hard to detect because hop latent viroid lies dormant in the plant until it blossoms, according to Leafworks, a plant genomics company that conducts research and developes commercial DNA testing services for the cannabis and hemp markets.

Joy Panyanouvong of Doc & Yeti Urban Farms, a licensed cannabis producer, trims marijuana plants in Tumwater, Wash., on March 15, 2023. Along the West Coast, which has dominated U.S. marijuana production from long before legalization, producers are struggling with what many call the failed economics of legal pot…a challenge inherent in regulating a product that remains illegal under federal law. (AP Photo/Eugene Johnson)