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GL1008-tim-hearden-microshoots.jpg-JM.jpg Tim Hearden
Wine grapes grow in a Sonoma County, Calif., vineyard. Growers are using microshoot tip therapy to cultivate virus-free vines.

Microshoot tip therapy helps ailing vineyards

The process has been used for decades on crops ranging from berries and citrus to fruit trees and grapes.

Well, shoot.

Or in this case, microshoot—specifically microshoot tip therapy.

What is it? It’s the process of culturing microshoot tips from an infected plant (read: grapevine) to generate a population of new plants using tissue culture techniques.

Sue Sims, a staff research associate with Foundation Plant Services, helped the National Clean Plant Network write the fact sheet on the viral elimination process under the heading, “Start clean, stay clean.”

“Basically, microshoot tip therapy takes the smallest growing tip you can manage to excise out of a plant and then have it grow into a whole new plant. During that process, because viruses do not infect cells as fast as they grow in the shoot tip, most of the time you lose the virus infection in the process,” she said.

Of the several types of therapies used to eliminate viruses and viroids from a plant, microshoot tip therapy is one of the most reliable methods, used for decades on crops ranging from berries and citrus to fruit trees and grapes.

Even the fact sheet admits there are still some mysteries to be solved. “It’s not known exactly how microshoot tip therapy actually works,” but it has been proven effective. “The success rate in removing viruses is different with different types of cells, varying by crop species, cultivar, and virus, but on average runs about 85%. If a plant is only infected with leaf roll virus, that success rate can jump as high as 99%.”

The process works well as a deterrent against the leaf roll viruses and seemingly so against red blotch virus.

Its success rate depends on which virus the plant is infected with because some grow faster than others and some hang back, according to Sims. Grapes take an average of seven months to grow from a tiny microshoot tip to a 6-centimeter plantlet.

Variables for success

There are a number of variables that help define success. “The cultivar makes a huge difference on whether or not the plant will grow in tissue cultures,” she said. “For instance, Cabernet Sauvignon is relatively easy to get tips to grow—often more than 90%. Sometimes they die because somebody didn’t slice the cut in the right way or the cells were just not wanting to live that day.

“With other varieties, especially some of the less vigorous kinds like pinot noir, the survival rate of the excised tips is much lower. The process faces two major factors that affect that average 85% success rate—getting the tip away from the virus and then getting enough of them to grow without the virus.”

So, what does this slice of science mean to growers?  In a year when many of the industry experts are advocating removal of less-healthy, lower-producing vines, it’s a chance to start anew with healthy stock.

“There’s plenty of research that shows if you get a plant that doesn’t have a virus infection, it will produce better grapes, more of them, and with less acid and higher sugar content,” Sims said. “For growers, just knowing about the availability of this process might reinforce their desire to use certified planting materials.”

In many cases, once a plant or a vineyard has a virus infection, cures are limited to non-existent.  “There are no viral compounds that work on plants in this kind of situation, so replacement of the vines may be called for,” she said.

Sims points to developing research that shows many of the common viruses are spread by mealy bugs that get blown around by the wind. “If your neighbor’s vineyard has a virus, even if you’ve planted clean certified material, within a few years mealybugs can get blown into your vines.

"We did a study that showed, on average, the infection rate was like 10% a year, so within five years at least half your vineyard could be infected. A regional approach among vineyard owners is recommended to minimize infection, with all growers planting certified materials, replanting with clean stuff especially around the edges of vineyards. You can control the virus this way by working together in a regional approach.”

For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line

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