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Cracking Grapes

Why are your grape berries cracking and what can you do about it?

Grape berry cracking is a process that involves the rupturing of the outer berry cuticle. This leads in later stages to the splitting of the fruit, which damages the produce substantially. The extraction of the high sugar content leads to the attraction of insects and disease pathogens, which feed on the sugary extract while causing damage to more berries and creating a “cascade of damage”. The berry rot expands to adjacent berries, resulting in a severely damaged berry cluster.

Grape berry cracking is a major problem in the table and wine grape industry. It causes increased expenses during harvest combined with yield losses, quality reduction and a shorter storage life.

Over the years a variety of explanations have been offered to this phenomenon, from incorrect nutrition, and insufficient calcium in the berry cuticle cells, to incorrect application of plant growth hormones or other chemicals that damage the cuticle and weaken it. But most researchers and growers agree that water, whether it is supplied by irrigation or from rain, plays a major factor in the damage.

 

What does the veraison have to do with the cracking?

Berry cracking usually occurs between veraison and harvest, in a period in which the levels of organic acids in the berry declines, sugar levels increase, and the berry goes through rapid growth.

Cell enlargement requires inner cell turgor pressure, and that pressure is triggered by the increased sugar content that causes water to enter the fruit due to increasing water potential differences with the outer cell solution. As the sugar content increases so does the water potential difference, pulling them in stronger and stronger.

In addition, the berries that were very firm until veraison – start to soften as they get closer to harvest.

The combination of increasing sugar content in the berry leading to increased water uptake, rapid growth and softening of the berry peel, creates a situation in which the berry is becoming more sensitive to cracking.

Under these conditions, a fruit that lacks essential structural nutrients that makes it weaker (or is damaged by incorrect application of chemicals) will be more likely to react to a sharp increase in turgor pressure by creating small cracks that are known as “mini cracks” that will eventually become major cracks and lead to the unwanted “cascade of damage”.

Therefore, it is no surprise that rain events have been known to be connected to berry cracking occurrence, as rain introduces a large amount of water with no salt content allowing the plant to absorb them easily into its systems.

When the plant absorbs a lot of water with low salt content, the water potential difference between inside the berry cells where there is a high soluble concentration (sugars) and the plant systems increases drastically. That creates a higher flow of water into the berry cells, causing the berry to expand more than it would probably do on a regular day, putting more strain on the berry cuticle.

 

An example of a drastic change in grape berry growth rate due to two rain events. The growth rate during the rain increased 6 times, from 0.1 mm a day to 0.6 mm a day. This sudden growth change generates a large strain on the berry cuticle and can lead to its splitting. (Tawny table grapes -SupPlant South Africa 2021)
An example of a drastic change in grape berry growth rate due to two rain events. The growth rate during the rain increased 6 times, from 0.1 mm a day to 0.6 mm a day. This sudden growth change generates a large strain on the berry cuticle and can lead to its splitting. (Tawny table grapes -SupPlant South Africa 2021)

 

 

What can you do to prevent the cracks?

There is not a lot that can be done during a rain event. It is not practical to try and prevent the rain from getting to the roots. Most efforts by farmers are aimed at protecting the berries from direct contact with the rain, to prevent direct water uptake and accumulation of access humidity on the berry that may increase sensitivity to disease pathogens later.

Still not all is lost, and a lot can be done to minimize the sensitivity of the berries to the sudden strain during a rain event.

One major factor in determining if a berry would crack during a rain event is the wholeness of the berry cuticle and the presence of “mini cracks” that create weakness points in the cuticle. Those may “give in” to the tension imposed on them during a rain event.

At SupPlant we have invested considerably in research on this subject in recent years and we have discovered that irrigation, and specifically irrigation in relation to day-to-day changes in climatic conditions plays a major role in preventing the occurrence of the “mini cracks” and reduce the risk of rain-related berry cracking. As a result, in the last years we had repeated success in helping farmers reduce the occurrence of this problem.

Not all farmers suffer from berry cracking due to rain events. Incorrect irrigation during berry maturation and closer to harvest can cause berry cracking in many sensitive varieties. That happens even in areas with hot dry summers, where berry maturation occurs outside the rainy season. There too, we learned that it can be reduced considerably from non-economical levels to manageable and sometimes negligible proportions.

To do so we combine data from our plant sensors, fruit sensors, soil sensors and meteorological data, and aim to maintain a stable fruit growth, with less sharp changes in berry size due to movement from heat waves to regular weather. We recognize the conditions that we know may increase the creation of “mini-trucks” at early stages and the bursting of those “mini-trucks” closer to harvest and adjust the irrigation regime in the sensitive period to reduce this phenomenon.

 

Nitzan Shatzkin, Chief Commercial Agronomist, SupPlant

 

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