Old Virus, New Crop: BCTV is Back

What’s old is new again with a reported resurgence of Beet Curly Top Virus (BCTV), a plague from Mexico to Canada “spreading havoc in over 300 species in 44 plant families” for more than a century, according to the agriculture folks at New Mexico State University. And it’s affecting cannabis and hemp.

The EPPO Global Database (European and Mediterranean Plant Protection Organization) lists the malady as “widespread” in nearly a dozen states, most of them in the west, while noting it is “present” in 10 other states.

In California, where the virus has hammered the sugar beet and tomato industry for decades, reports are being received that the disease, vectored by Circulifer tenellus — the beet leafhopper — has now discovered cannabis cultivators and has become hemp’s newest worst enemy.

“BCTV has an extensive host range, a high reproductive capacity, and can migrate long distances from its breeding grounds,” reports the pest management guidelines published by the University of California.

_{Beet Leafhopper}

Leafhoppers tend to overwinter in annual and perennial weeds and acquire the virus when they feed on infected plants. And it doesn’t take long. Say pathologists: “Leafhoppers are effective vectors able to transmit the virus after feeding on an infected plant for as little as one minute.”

Transmittal to nymphs comes when leafhoppers feed on infected plant phloem. Once the leafhopper ingests the virus, it moves from their digestive tract to the salivary glands and the virus is transmitted by eating more phloem.

“Once acquired,” reports the University of California Statewide Integrated Pest Management Program, “the vector can transmit the virus for the rest of its life, although the effectiveness of transmission is reduced when the insect does not continually feed on infected plants.” If there is any kind of bright side involving the carrier mechanism, it’s the fact the virus is not passed on to leafhopper progeny.

When BCTV arrives, it doesn’t paint a pretty picture. Broad-leaf plants ranging from tomatoes and peppers to cabbage and cucurbits — and now cannabis — become dwarfed, crinkled, and rolled upward and inward. Veins on infected leaves are swollen with bumps and rootlets die off after becoming twisted and distorted.

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Fruit is deformed (quality and yield reduced) as leaves turn yellow with purple veins. While leaves curl upward and become thickened, petioles curl downward.

Josh Schneider runs Cultivaris Hemp LLC in San Diego and, as a plant pathologist, explains things this way: “A virus is an inanimate snippet of DNA or RNA that uses the cell of other plants to replicate itself. In principle, a virus infects a cell and forces the cell to produce countless more copies of the virus, copies that infect other cells in a process that repeats itself.”

There are virus issues with clonal crops, especially in the case of cannabis grown for female flowers. Adopting clonal cutting production to ensure all-females with precise yield and active ingredient content “poses considerable, inherent, risks,” he says.

_{Photo courtesy of University of California}

“In today’s cannabis industry, clonal material is traded and swapped between production sites. Given how widespread certain virus diseases are, and how all clonal progeny from an infected plant are likewise infected, this scenario is a potential catastrophe in the making, and we’re only beginning to learn enough about cannabis susceptibility to a vast range of diseases, viral and pathogenic.”
Schneider is a purist when it comes to clean stock, so he believes the leafhopper is getting a bad rap in the California outbreaks.

“Where do most hemp plants in the US come from?” he asks. “Colorado, ground zero for domestic hemp for the last five years. And guess who grows a lot of sugar beets? Colorado. My best guess is that infected clonal material was distributed around and transmitted during the propagation of young hemp plants.

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“In order for the beet leafhopper to be the main culprit, it would have had to have fed on beet plants, then fed on cannabis plants, and then it would take a while for the virus to get circulated through the plants. Many viruses take a number of years, so it’s hard to give a pithy soundbite because there are so many variables like the time between infection and conversion where the virus is being fought, but symptoms are not visible.”

Further up the coast of California, plant pathologist Bob Gilbertson at UC Davis offers some advice to mitigate the problem: “We don’t know much yet about how different cultivars of hemp will respond to any resistance, so the best approach is going to involve timing and location of planting.

“If you plant near where leafhoppers overwinter, you’ll have a much higher probability of having a curly top virus problem because they’ll quickly spot your crop. You could also plant either very early or later in the year to avoid leafhopper migrations which occur in March and April. Because hemp needs to be organic, growers can’t use pesticides, so location and time of planting are the two things that can be done currently.”

Schneider is a bit more sanguine about the situation: “I don’t think this is a Chicken Little sky-is-falling problem. Good sanitary practices will help prevent it and good pest management practices will help control it. My best advice is to start out with clean stock.”