Purple weed could save California’s pot farms from ‘severe’ pathogen

By Lester Black, SFGate

FILE: A purple cannabis plant grows at an outdoor farm. There’s evidence that purple varieties of cannabis might be able to fight a severe crop disease currently affecting pot farms.

An estimated 90% of California’s cannabis farms are infected with a “severe” pathogen that can destroy the value of pot plants, leaving cannabis farmers with a worthless harvest. But a recent scientific breakthrough is offering a glimmer of hope for the beleaguered farmers.

Scientists have discovered a type of purple-colored cannabis that appears to fight the widespread plant disease, which is called hop-latent viroid, or HLVd. HLVd damages cannabis plants and significantly reduces their value by decreasing the amount of active compounds, like THC, they produce.

But two weeks ago, a group of scientists at Medicinal Genomics, a Massachusetts-based company, announced they had accidentally discovered that one of their cannabis plants appears to be partially resistant to HLVd. The scientists also noticed that the plant turned purple as it was fighting the disease. Cannabis plants can occasionally turn from green to other colors, like red, blue or purple, because of genetic or environmental factors. Purple plants have long been valued in the cannabis community, with prized strains like Granddaddy Purple, Mendocino Purps and Purple Haze. Kevin McKernan, the chief science officer for Medicinal Genomics, announced his company’s findings earlier this month at a conference in Florida. McKernan said the plant was significantly more purple than a second plant that had the same genetics but was not exposed to the viroid. “We don’t know why [this is happening]. This could be an immune response, but we’re not seeing this [purple coloring] as heavily increased in the control that’s not infected,” McKernan said. A ‘major threat’ to cannabis farms Zamir K. Punja, a Canadian professor of plant biology, said at the same conference that he considers HLVd to be a “major threat” to cannabis farms. He said his research has found that infected plants can reduce their THC yield by 40%, causing severe economic damage to a farm. It’s extremely hard to get rid of the pathogen because HLVd spreads easily between plants, sticking to tools and circulating inside a pot farm’s water supply. Not only that, but once a plant is infected with HLVd, it makes the plant more susceptible to catching other diseases — a phenomenon that led Punja to dub HLVd the “COVID of the cannabis world.” Marijuana leafs glow purple under grow lights at the Black Dog LED booth during the High Times Cannabis Cup at Denver Mart in Denver on April 19, 2014.Seth McConnell/Denver Post via Getty ImagesThe good news is HLVd does not pose any health risk to humans because it strictly affects plants, Punja said. He added that there is evidence that some varieties of cannabis are much more resistant to the effects of HLVd. “There are plants or cultivars or chemovars that differ in their ability to withstand this viroid. … Some can fight this thing off much better than others,” Punja said. Punja’s analysis combined with the recent discovery at Medicinal Genomics begs the question: Could California’s farmers really be saved by purple weed?

How to fight HLVd McKernan said his company didn’t originally realize they had a plant with a unique response to HLVd. The scientists were intentionally infecting plants with HLVd to try to understand how the disease changes infected plants. They rubbed the viroid directly onto cut leaves of the plant, an established method for infecting plants with diseases. But then they ran into a surprising problem. One of their specimens, a variety of cannabis they call Jamaican Lion, “would not infect,” according to McKernan. The team repeatedly exposed the plant to HLVd over the course of six weeks, but the plant never tested positive for the virus. Finally, the scientists injected the viroid directly into the stem of the plant, but even then the disease appeared to only infect the plant’s roots and not any of the leaves. The team then noticed the infected plant’s leaves and flowers were turning purple, but a second uninfected version of the same plant was not changing color. They continued to test the plant for HLVd, but 57 out of 57 tests on purple plant tissue turned up negative. The only leaves that ever tested positive for the virus were green leaves on the lower part of the plant. FILE: Sustainable cannabis farmer Dylan Turner applies fertilizer to a crop of plants at Sunboldt Farms, a small family farm run by Sunshine and Eric Johnston in Humboldt County, Calif.Robert Gauthier/Los Angeles Times via Getty ImagIt’s not clear why the purple plant material is associated with fighting the disease. McKernan attributed the color change to “jacked up” production of anthocyanin, a plant chemical that can turn cannabis purple. McKernan said it’s worth investigating more purple plants to see if they are tolerant of HLVd, because anthocyanin production is already linked to fighting viroids. “There is literature that links Anthocyanin production with viroid infection. This is in different plants and with different viroids but these anthocynanins are a known immune response for plants,” McKernan told SFGATE in an email. McKernan added that his company is running new experiments on their Jamaican Lion plants to better understand how the variety fights HLVd. Cannabis farmers are currently fighting HLVd with a combination of cleaning techniques and constant testing, in the hope that they can remove infected plants and reduce the spread by cleaning their farms. But McKernan said at the conference that his company’s findings show that cannabis farms will need to use new breeding technology to fight this disease and not rely exclusively on cleaning agents. “You’re not going to bleach your way out of this problem, you are going to have to breed your way out of this problem,” McKernan said.