Hot water treatment of grapevines is an important—and in some cases, mandatory—tool for controlling the spread of pests and disease, but for some growers the cure is worse than the disease.

Many growers have reported that vines treated with hot water exhibit reduced vigor, poor growth, dying leaves, and delayed bud break, said Dr. Pat Bowen of the Pacific Agri-Food Research Centre in Summerland, British Columbia, Canada. Bowen made a presentation to the annual viticulture and enology conference the British Columbia Wine Grape Council hosted in Penticton, B.C. this past ­summer.

The vigor problems led some growers in Canada to oppose a recent federal requirement that came into effect in 2006 requiring hot water treatment of all vines imported from Europe in order to combat two diseases not yet found in Canada: flavescence dorée and bois noir. The resistance in turn led Bowen to investigate
the phenomenon to figure out why the ­problems were occurring.

"There was research lacking. If done properly, if you look in the literature, it looks like it doesn’t have a very big effect on the vine," Bowen said. "It certainly shouldn’t lead to the effects that growers are finding in the field."

Bowen’s study looked at the response of Chardonnay and Merlot vines to hot water treatment, which involves placing vines in water heated to 50°C (122°F) for 35 minutes. Trials took 800 vines of each variety and looked at the effects of hydration, acclimation between cold storage and treatment, the immersion process, and various lengths of cold storage.

The five sets of variables (and combinations thereof) aimed to determine if the troubles afflicting treated vines were the results of a break in dormancy or tissue damage as a result of the treatment.

Physiologically, Bowen thinks tissue damage is the most likely explanation.

"If done properly, if you look in the literature, it looks like it doesn’t have a very big effect on the vine; that the hot water, the heat, could be killing tissues or even individual cells in tissues, or denaturing proteins or enzymes which are absolutely needed for development," she said.

Supporting this conclusion was a pronounced delay in bud break in both the Chardonnay and Merlot vines—typically between four to six days—and diminished vigor, particularly in the Merlot vines.

While the troubles growers face are real, Bowen doesn’t believe the research points to a severe problem. The effects did not carry over into the second year after planting.

"The delay in bud break, we think, is pretty trivial compared to the length of the growing season and the choice of planting dates. And it’s very similar, but a little less in magnitude, to what you find with cold storage, and nobody seems to be that concerned about the effects of cold storage separated from planting date," she said. "Devigoration was ­certainly there, but not severe."

So what’s leading growers to believe hot water treatment leads to poor-quality vines?

Allan Schmidt of Vineland Estates ­Winery in Vineland, Ontario, suggested to Bowen following the start of the study that the woes might be the result of vines being placed in unperforated shipping and storage bags in France that effectively asphyxiate vines wakened from dormancy. Bowen thinks this might be possible, though her study used only perforated bags.

"It just never occurred to us that they weren’t perforating the shipping and storage bags," she said. "When the vines are fully dormant they have very low requirements for oxygen, but if you’ve hot-water treated them, woken them up, suddenly they’re metabolizing then, they’re respiring, and they’ll need oxygen. It could be that they’re starved for oxygen."

She encouraged growers to perforate the bags in which vines are stored, and when the time comes to plant, to plant early so that vines have time to recover from any lingering effects of cold storage. 

HOW ARE VINES treated?

One of the big variables for growers working with vines imported from Europe is the way vines are handled prior to shipment.
"We don’t know how they’re managing the hot water treatment in Europe," said Dr. Pat Bowen of the Pacific Agri-Food Research Centre, located in Summerland, British Columbia, Canada.

The comment prompted a response from Lloyd Schmidt of International ­Viticulture Services, Inc., in Grimsby, Ontario, who backed up Bowen’s suggestion that prolonged treatment in hot water may be damaging vine tissues and in turn contributing to diminished vine performance the year after planting.

Schmidt said workers in France typically place vines in water once it reaches 50°C (122°F). Since this usually cools the water a fraction, the water is allowed to return to 50 degrees before timing of the treatment begins. The result is that the vines may be in hot water for up to 48 minutes, rather than the 35 minutes ­prescribed.

Workers put the vines back into cold storage four or five hours after treatment. Many of the vines stored in plastic developed a fermentationlike problem, Schmidt said, with the vines acquiring a slimy feel and smell.

"This was not the right way to do it," he said. "Hot water treatment has worked much, much better. I don’t believe there have been any problems in the million vines we have treated."

Schmidt says it’s important to get the process right, because it has shown itself to be an effective preventative measure that’s attracting a growing following ­worldwide. —P. Mitham