A mysterious syndrome that leads to cracking on grapes is still under investigation by University of California researchers. Under current study is whether Ethephon, a spray used to promote berry color, is largely responsible for grape berry cracking.

Grape berry cracking is not a new phenomenon. It has been found for more than two decades, according to Mark Matthews, professor and plant physiologist in the Department of Viticulture and Enology at the University of California, Davis.

Matthews discussed grape berry cracking in Fresno, Calif. in June during a San Joaquin Valley Winegrowers Association meeting. He told growers that for all the work that continues on the subject, a clear cause of grape berry cracking still eludes researchers.

“We’re not clear on whether cracking comes before bunch rot or if bunch rot comes first,” Matthews said. “It’s also not clear how we manage it.”

Grape berry cracking does not seem to be a widespread problem, according to Matthews. Flame seedless table grapes and Zinfandel wine varieties appear to have higher rates of cracking.

“One of our challenges in this study was to go out and see if we could make some cracking happen, and see if we could manage that,” Matthews said.

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Initial studies into grape cracking started with the hypothesis that higher irrigation rates were responsible for grape berry cracking. Research in the 1980s suggested this and was the direction Matthews was working when he had his mind changed from his own Ethephon results.

“These Ethephon results really took us in a little different direction,” he said.

Cracking tends to appear after veraison, the period when ripening begins in the grape. This also happens to be the same time when Ethephon is traditionally used.

According to Matthews, the impact of Ethephon on berry cracking is almost immediate.

“Within 20 minutes of exposure to Ethephon the berries became weaker and were susceptible to cracking,” he said.

Studies show that Ethephon decreases berry firmness, which leads to cracking, but has no impact on berry size.

While a promising breakthrough in grape berry cracking studies, the Ethephon trials yielded conclusive results in only two of three years of field trials. The third year showed no significant differences between grapes sprayed with Ethephon and those in the control vineyard.