Farm Progress

Mystery "residue scab" puzzles researchers

September 4, 2007

2 Min Read

An anomaly of some kind showed up on wine grapes in San Diego County earlier this spring, but no one seems to have pinpointed the exact cause.

It has been described as a “residue scab” by Pete Anderson, a private researcher who operates a small research vineyard at Carlsbad, Calif.

“It first appeared on berries after set,” he says. “The residue scab was removable by rubbing early in the season, but then became hardened and more attached to the peel. I compared it to thrips damage and measles, and it is not similar to either.

“There was some black spotting on the canes, but not on all. There was no leaf damage. It’s very strange. I gave a talk in the county earlier this spring and asked if anyone else had seen it. About 15 people raised their hands.”

At first, the consensus was that it was some sort of chemical residue. According to Anderson, the problem with the chemical residue theory is there was no common thread among other growers in the county who saw the same problem.

“I use mostly JMS Stylet oil to control powdery mildew and leaf pulling to manage Botrytis, but other growers who saw the same symptoms were using different programs,” he says.

UC researchers have yet to identify the specific cause of the problem.

One of the theories being bantered about is the quirky year in terms of weather.

“I had six days of freeze (short-lived each day) and only three inches of rainfall,” Anderson says. “There were several vines that didn’t produce bud break on some spurs and canes. At first I thought it was varietal-specific, since 100 percent of the Sangiovese vines were affected by the scab. Then I discovered some Cabernet Sauvignon and Mammalo Toscano also had some.”

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