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Seed growers cooperate to slow plant virus

Issue Date: January 27, 2021
By Kevin Hecteman
Syngenta agronomist Brandon Hunt describes machinery used to clean watermelon seeds after harvest in Sutter County. After detections of cucumber green mottle mosaic virus in Sutter County and elsewhere, a Clean Seed Agreement was drawn up, under which only cucurbit seeds certified to be free of the virus are to be planted. Cucurbit crops susceptible to the virus include watermelons, cucumbers and cantaloupes.
Photo/Kevin Hecteman

The rate of cucumber green mottle mosaic virus in Sacramento Valley fields is low—and seed growers intend to see that the plant disease never establishes itself in California.

The virus affects cucurbit crops such as cucumbers, watermelons and cantaloupes, and can lead to reduced yields and crop quality. Mottled leaves could be a sign of the virus.

Dennis Choate, who works as a vegetable seed production manager for Syngenta in North America, said the U.S. Department of Agriculture calls the virus a quarantine pest, "because it's not supposed to be here in the U.S." He noted it appeared in a seed field in Sutter County in 2012.

Following that first discovery, Choate said, a handful of detections followed in the next few years. Then, in 2017, seven positive detections occurred in seed-production fields of triploid watermelon and cucumber in Colusa, Glenn, Sutter and Yolo counties, along with one potential find in squash in Fresno County.

That jolted seed companies and growers into action.

"At that point, the industry got together and formed a working group through the California Seed Association to begin to address what was becoming a problem for us," Choate said, adding that the association also launched a grower-awareness campaign to outline best practices.

Those efforts led to a Clean Seed Agreement— sponsored by the seed association and implemented last year—and a set of hygiene protocols for people and machinery intended to eliminate virus transmission from field to field.

"We got the various seed companies that are growing cucurbit seed in the Sacramento Valley to agree to only plant seed that was tested clean and free of CGMMV," Choate said of the agreement.

Detections of cucumber green mottle mosaic virus in California have dropped from seven in 2017, and perhaps as many in 2018, to one each in 2019 and 2020, he said.

Patty Buskirk, who runs a seed company and manages a row-crop seed-production farm in Colusa County, said bringing infected seed to an area "could jeopardize a grower base" and that "it's important for all of us to regulate ourselves, to make sure that this quarantine disease doesn't continue to grow."

"There isn't any seed that leaves my warehouse that gets planted for mother seed, for a seed-production plant or field, that hasn't been tested," Buskirk said. "I don't know of a nursery in California that will even let you plant a cucurbit seed in their nursery unless you give them a negative test with a lot number on it."

Hygiene measures for people and harvest equipment are part of the process, Choate said.

"We know that CGMMV can very easily be carried through mechanical transmission—people touching the crop, the equipment in the fields, going from plant to plant to plant, any kind of pruning activities that may happen in the course of seed production," he said.

To combat this, crews enter a field through a designated site and walk through a foot bath to disinfect footwear.

"In the case of hybrid watermelon production, the pollination process is extremely hands-on," Choate said, with "up to 100 people pollinating at a time, depending on the size of the field. Getting those crews to only be in that field, and completely sanitized before they went to another field, was a key component."

The same goes for greenhouses where cucurbit seeds are sprouted before being transplanted: "In some of the breeding efforts, if you're using scissors or some of the tools like that, you're sanitizing from plant to plant, even," he said.

Syngenta agronomist Brandon Hunt said watermelon seeds are washed in the field at harvesttime, and again at the farm, before being sent to a facility for a final cleaning before distribution. Harvesters also are thoroughly sanitized before leaving the field, he added.

"It could be anything that can move it," Hunt said. "We don't really even know yet. So we're trying to prevent everything."

Buskirk said she was first alerted by a seed-company representative about the issue and put her plant-pathology education to work researching the virus.

"Since we're a seed-production region and we're exporting in the Sacramento Valley and Northern California, we are even more interested and vigilant to be able to do our seed production here," she said.

The Sacramento Valley has been a seed-production hub for some 80 years, "and we wanted to keep it that way," she added. "That's the reason to be very, very, very diligent."

Tera Pitman, a staff research associate with the University of California, Davis, Department of Plant Pathology, said research into the virus has involved looking into alternate weed host possibilities and the genetic variability of the virus that's been found in California.

"The project that's currently ongoing is sequencing isolates that have come in with seed shipments," Pitman said, "just to get a clearer picture on where these things are coming from and how much genetic variation is out there."

The likeliest scenario, she said, is that "it has been introduced multiple times with seeds entering California." Tested seeds have shown similar sequences to those found in Europe, India and China, she added.

Pitman said growers who think they have a CGMMV issue can use Agdia ImmunoStrips for field identification; if it's not identified in the field, growers should contact a UC Cooperative Extension farm advisor and the California Department of Food and Agriculture.

A positive detection in the field results in a three-year remediation process, during which cucurbits can't be grown in that field and growers need to suppress weeds as much as possible, Pitman said. As the virus only affects cucurbits, other crops can still be grown.

"As long as your crop isn't removing soil along with whatever it is you're growing, then it's not a big deal," she said.

When the three years are up, growers returning to cucurbits need to ensure they're buying seeds certified to be free of the virus, Pitman said, noting that while research into potential resistance to the virus has been done, no resistance has yet been found.

(Kevin Hecteman is an assistant editor of Ag Alert. He may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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