What is in this article?:
- Primary cause of grape cracking still elusive
- Acidic conditions
- Ethephon spray appears to be a cause of grape cracking in early University of California grape trials.
- Irrigation practices may play a role in grape cracking, but not in ways previously thought.
- pH levels on grape skins may be a cause of grape cracking.
Mark Matthews, plant physiologist with the University of California, Davis, explains how various cultural practices may lead to grape-berry cracking.
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.
More stories from Western Farm Press