What causes shrivel?
Researchers thought at first that climate might be involved.
Researchers in British Columbia aren't shrinking from the study of berry shrivel, a disease that's been growing in significance in the Canadian province in recent years.
Present throughout the world's major wine regions, the disease has no known cause or discernible pattern of occurrence. It appears after veraison, arresting the development of grapes and leading to berries that look like deflated soccer balls. Diseased fruit is pale in color, has a sugar level of less than 16 degrees Brix with high acidity. Affected grapes taste sour.
Whole clusters and single berries may be affected, but a vine with berry shrivel one year may be free of the disease the following year. The erratic nature of the disease makes it expensive to root out because workers must inspect clusters individually before harvest, boosting labor costs.
Growers typically notice the disease when it affects 3 percent of a vineyard. It's considered a serious problem when 10 percent of vines are affected, at which point it reduces crop volumes and threatens wine quality.
Federal research technician Carl --Bogdanoff has been working with Dr. Pat Bowen and other researchers at the Pacific Agri-Food Research Centre in Summerland, British Columbia, since 2005 to pinpoint the disease's roots.
The research is studying six vineyards growing Sauvignon Blanc, Semillon, Pinot Gris, Pinot Blanc, and Cabernet Sauvignon grapes—varieties that appear to be more susceptible to berry shrivel than others.
The researchers initially looked to find a relationship between climate and the onset of the disease. They also tracked levels of the plant hormone gibberellic acid (which plays a role in plant development), and data associated with light exposure, irrigation practices, and shoot and cluster management.
But Bogdanoff told growers attending the B.C. Wine Grape Council's annual viticulture conference in Penticton last July that preliminary data indicates that climate is not a significant factor.
"We couldn't find any specific climate event that triggered berry shrivel," he said.
While nutrition and irrigation don't appear to play a role in the onset of berry shrivel, Bogdanoff said there appears to be a relationship between the number of shoots per vine, exposure to sunlight, and crop load.
"Shoots with two clusters had more berry shrivel than shoots with one cluster," he said. "Not-thinned vines had way more berry shrivel than cluster-thinned vines."
There also appears to be a correlation between the amount of photosynthates flowing to the grapes and the incidence of berry shrivel.
"A major component would be the carbohydrate load, or the amount of photosynthates produced in a shoot that provide the resources for that cluster," Bogdanoff explained following his presentation. "To us, this is a sort of resource partitioning in the plant, where the plant somehow decides it's going to abandon those clusters on those shoots. Again, it could be just one cluster per vine, or several clusters per vine, or on some vines and not others in the vineyard. It's really sporadic."
Bogdanoff said future research would examine shoot removal and cluster thinning, to see if these have any impact on the disease. Other factors will continue to be tracked.
Bogdanoff added that one potential factor may be the plant hormone gibberellic acid, which controls growth and the flow of nutrients in a vine.
"GAs are produced by the seeds in the fruit, so we're thinking it has to be important," he said, noting that it may play a role well before veraison, with the effects seen much later.
"We don't know what the cause of berry shrivel is, but it looks like there are many factors involved," Bogdanoff told growers.