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.

Acidic conditions

Some interesting insight came from Matthews’ work when researchers discovered that the effects of Ethephon are pH dependent. Tests showed that over time Ethephon decomposes into ethylene gas, chloride, and phosphoric acid. As the pH dropped so did the time required for berries to crack.

Students who worked with Matthews during the summer discovered that buffer agents combined with Ethephon greatly increased the time between application and when cracking appeared. These buffers tend to help increase the pH on the berry skin, which has shown in lab studies to increase the time between exposure and cracking.

“One of the things we’ll be doing this year is to buffer the Ethephon spray to reduce cracking,” Matthews said.

Grape-berry cracking appears to be the primary cause of bunch rot, according to a 1980 study by M.J. Barbetti on Australian wine grapes. Yet Matthews says little is known about it in the U.S.

What is known is grape berry cracking leads to yield loss and increased packing costs for growers by the removal of cracked grapes prior to shipping.

Of primary focus are a variety of cultural practices which Matthews believes may lead to cracking. These appear inconclusive in the cause and effect as conventional wisdom has not yielded positive results.

For instance, girdling vines to increase berry size or improve berry color was thought to have an impact on grape cracking based on 2010 studies. Follow-up studies the next year showed little difference between girdled vines and those not girdled.

The conclusion was girdling did not have a clear impact on cracking.

Additionally, irrigation practices, thought to be a leading cause of cracking, appeared to defy hypotheses. It was once thought that heavy irrigation practices led to cracking since internal grape pressure increased with water applications. As it turned out, vines stressed by too little water were more susceptible to cracking.

“To our surprise, we had a lot more cracking (from water-stressed vines) and not a lot less,” Matthews said. “Under irrigation may be of more concern than over irrigation.”

Also interesting is a hypothesis that cracking is initiated by local stressors in the grape skin, and not an overall expansion of the berry created from internal pressures as is suggested by previous research Matthews referenced for his studies.

This work suggests that some grapes simply have a thin skin which leads to cracking when pressures from within the grape drive expansion and exceed the physical properties of the skin.

As his work continues, Matthews’ observations suggest the previous thin-skin hypothesis by other researchers may not be the case. Some other more local phenomenon is occurring on the berry, which appears to lead to cracking or tearing, rather than an overall force exerting pressure against the skin from within the grape.

Matthews discovered this through cell-level microscopic studies of grape skins. He found that tears, rather than take place where individual cells bond together and one cell tears away from an adjacent cell – were a result of individual cells tearing or splitting in two.

Something is happening within the cell structure itself, he said, which is causing the cells to tear. He still does not know what causes this.

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