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Curly top virus in hemp UCANR
Hemp plants in California are infected with the beet curly top virus.

Old virus finds a new victim -- hemp

Beet curtly top virus initially discovered more than a century ago

Beet curly top virus certainly isn’t new, but it has found a new victim amidst the increasing popularity of the cannabis industry.

Initially discovered well over a century ago in the Western U.S. and ranging from Mexico to Canada, BCTV increasingly was responsible for loss of yield with many crops destroyed by the disease. BCTV is listed as ‘widespread’ in places like California, Arizona, New Mexico, Utah, Washington, Oregon, and six other states, and classified as ‘present’ in 10 other states.

The pathogenic plant virus belongs to the family Geminiviridae consisting of nine genera with BCTV being a Curtovirus affecting hundreds of plants.

“The virus has a wide host range, spreading havoc in over 300 species in 44 plant families,” according to the College of Agriculture at New Mexico State University, further noting: “The virus appears restricted to broad-leafed plants, the most-commonly infected hosts being sugar beets, tomatoes, peppers, cucurbits, cabbage, and many ornamentals.”

AFFECTING HEMP

It has now discovered the growing popularity of hemp cultivation in the West after the beet leafhopper vectored the disease into the tomato industry a few years back. Back then, Bob Gilbertson, plant pathologist at the University of California, Davis, called the disease “perplexing to work on” because combatting the leafhopper is no easy task as it migrates quickly.

In San Diego, plant breeder Josh Schneider, CEO and founder of Cultivaris Hemp LLC, says viral diseases are causing more and more catastrophic losses and BCTV potentially represents a bugaboo for marijuana and hemp growers because “the more exposure a plant has to a virus, the more the virus adapts to infect the plant more readily.”

Originally working in the ornamentals industry, Schneider found that viruses that originally attacked only one plant in a family, given enough exposure to other genera in production meant the virus migrated and developed a broader host range by the exposure.

“It makes sense that more virus infections are probable the more exposure the plant has to other hosts, so in Ventura County for example where they’re growing beets and other cruciferous stuff outside and you have a virus that infects them --- and there’s cannabis being grown in nearby fields --- insects and people are what move viruses and it’s hard to control either of them, especially in field production.”

ABOUT BCTV

BCTV is transmitted to nymphs of beet leafhoppers when they feed on the phloem of plants that are virally infected. The virus does not replicate within the leafhopper and causes them no harm. Likewise, it cannot be passed on to their offspring.

Once the leafhopper ingests the virus, it moves from digestive tract to salivary gland and as the infected hoppers migrate and feed on healthy plants, they transmit the virus by eating the phloem. Younger plants appear more susceptible to damage and by all appearances, BCTV tends to infect dicotyledonous hosts rather than monocotyledonous plants.

During the winter in California, the beet leafhopper migrates to the foothills of the coastal range on the west side of the Central Valley and lays eggs in the perennial weeds and buckhorn plantains. When nymphs are born, they consume the infected weeds and the virus is spread further as the new adults travel to agricultural areas of the Central Valley in late spring to infect other healthy plants.

“The leafhopper is an active flyer and as it comes down out of the foothills, flying around the valley floor, it looks for green patches as it visually senses for food,” says Gilbertson.

“It heads to tomato, beet, and now cannabis fields for a plant taste-test to determine if it’s the kind of host it wants to complete its lifecycle on. If it’s an undesirable host, it doesn’t stay long, but in the course of its short-term tasting that particular plant, it has fed long enough to transmit the virus.”

FALSE CONFIDENCE

Farmers sometimes have false confidence when it comes to BCTV because they don’t see any leafhoppers in their fields.

“That’s because the hoppers came, did a taste-test, and left before the farmer could spot them, but in the process, leaving behind the prospect of a loss of yield and quality of crop,” Gilbertson says.

Like any resurgence of a problem, it takes time to discover it, then figure out ways to remedy it. “It’s only in the last year that we first started seeing a lot of curly top showing up in hemp fields,” Gilbertson says. “We need to learn more about just how well the hoppers like hemp and what the viral affects will be at different stages of inoculation.”

As to what kind of year 2020 will prove to be: “It’s too early to predict because we don’t know how well the leafhoppers will survive in the foothills this winter. You need two things to predict how bad it will be next year --- how well the hoppers overwinter in the foothills and how much virus they’re carrying, and it’s usually January or February before we can make predictions of just how bad the curly top plague will be for 2020.”

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