The quality of table grapes postharvest is directly affected by practices in the vineyard, often long before the grapes are packed and shipped to export destinations.
This was one of the findings highlighted by presenters. The event was held in collaboration with the South African Table Grape Industry (SATI) and Raisins South Africa in August this year.
Presentations centred on efforts to enhance the postharvest quality of table grapes and, in the process, unlock and secure markets. Postharvest challenges and innovation in the sector also came under the spotlight.
In her presentation, Tinake de Klerk (Capespan’s technical manager) explored some of the reasons why export grapes that have been carefully packed still arrive with problems at their destination, despite producers’ efforts to use special packaging to extend shelf life.
Controlling what you can
There are many factors affecting grape quality that producers have no control over, stated De Klerk. It is impossible to predict how grapes will respond to shipping and how they will be handled once the containers arrive at their destination weeks later. Furthermore, even if producers provide clear guidelines for everyone along the supply chain, they have no guarantee that these will be followed.
“Let’s then focus on what we can control,” she said. “This includes how we prepare our grapes for harvest (weather permitting), how the grapes are harvested, and how they are packed.”
The aim of postharvest management must be to limit the rate of deterioration after harvest. But growers’ efforts to ensure grape quality should start long before harvest, she emphasised.
For instance, the ability to harvest grapes at the desired fruit ripeness depends on growers’ knowledge of the progression in fruit maturity occurring in the vineyard. Harvesting grapes at optimal fruit maturity or ripeness presents many challenges, including doing an accurate assessment of fruit ripening. De Klerk pointed out there is no clear guideline in the industry about what precisely ‘optimal ripeness’ means. Yet, this term is widely used to help determine when fruit should be packed for the export market. In addition, ‘optimal ripeness’ could mean different things in different regions and for different varieties, she stated.
This means that monitoring local vineyard conditions remains key. Weather conditions will cause seasonal differences in the rate and characteristics of grape ripening. Varieties and even blocks of the same variety are also likely to have different patterns of ripening.
Growers should consider the trends in their specific production area, including winter rainfall, humidity during bloom, and disease pressures, she said. By analysing weather conditions, growers can predict whether Botrytis may become a problem postharvest.
Always follow good sanitation practices in vineyards, De Klerk advised. This includes getting rid of sclerotia-infected material that could germinate in spring.
Many growers disregard the golden rule of not packing grapes immediately after rain. As such, De Klerk emphasised the importance of packing dry grapes to avoid problems at a later stage – grapes harvested after rain are much more prone to decay than those harvested after a dry period. If the grapes have been exposed to rain for more than 10 hours, packing operations should be halted for three days, she advised.
According to De Klerk, it has become increasingly evident over the last few seasons that there is no definitive answer to some of the questions around the relationship between postharvest management and grape quality. What is clear, however, is that efforts to ensure grape quality should start long before harvest. Therefore, producers should get the basics right if they are to successfully manage those factors within their control.
Boosting cell division
During the same session on postharvest practices, Trix Quixley (NexusAG’s technical and commercial manager) discussed factors that affect table grape quality, ranging from bud break to harvest.
Quixley said that, although all the different grape development phases are important, the stage starting at fruit set, when berries grow and undergo cell division, remains vital. This is because the number of cells formed during this phase later affects berry firmness, dryness, and its susceptibility to fungal attack and decay.
“Cell division is influenced by water conditions and optimal temperatures (20 – 25°C),” she said. “Water stress during the cell development phase causes fewer cells to form. As such, it is crucial to ensure optimal irrigation and root development during this phase,” she emphasised. She also advised growers to monitor growing conditions, bearing in mind that temperatures higher than 35°C lead to smaller berry size.
“Good vineyard management practices remain key to ensuring postharvest quality,” she continued. These practices include thinning out bunches, following a preventive Botrytis spraying programme, preventing physical damage to bunches (caused by thrips, fruit flies, sunburn, hail, birds and bollworms), and the removal of damaged bunches. “Good quality grapes start in the vineyard,” advises Quixley.
Over- and under-irrigation, for instance, cause water stress. This, she says, calls for growers to ensure optimal irrigation practices during the cell division phase in particular. Canopy management should also receive attention to prevent overshadowing as it leads to reduced cell division.
Quixley added that suboptimal growth conditions and poor root development during key growth phases also affect cell division. As such, stimulating root development and ensuring optimal conditions for root growth, including nutrient availability, remain essential to promoting grape quality. “And organic material should be distributed throughout the vineyard to help ensure a better spread of the vines’ fine hair roots,” she said.
Practices that can increase cell development include stimulating hair root development and active shoot growth, adding organic material to the soil, controlling nematodes, monitoring soil nitrogen levels, and thinning out bunches early on to remove competition between shoots and branches.
Quixley advised growers to implement and refine control measures to avoid pathogen-related infections during the growth and development phase (especially under high disease pressure and conducive conditions). She concluded by highlighting the importance of growers protecting the hair roots of grapevines by implementing proper nematode control and salinity and pH management in vineyards.
