Dealing with decay – Ensuring table grape quality

Curbing pathogen-related decay in table grapes

While decay symptoms in table grapes develop during storage, pathogen infections may start preharvest during flowering and during the growth and development phase (especially under high disease pressure and conducive conditions). Onset may also occur during or after harvest and packing, through the spread of inoculum among fruit or as a result of injuries sustained during the handling process.

According to Fourie, pathogen-related decay is “seemingly on the rise”. Climate conditions have also changed in key production areas: high temperatures and frequent summer rain spells have been documented in recent years.

Fourie outlined control measures to deal with pathogens ranging from vineyard practices before harvest (such as canopy management, good sanitation practices and fungicide applications) to practices after harvest (including in packhouses).

He urges producers to consider disease cycles and the level of decay present, and then to devise risk management strategies. “If decay increases, select venues with lower disease levels first for harvest and packing at that time.”

The cold chain must also be managed carefully, including during and after forced-air cooling, during shipment and distribution and in retail. Storage temperatures must be managed to avoid the spread of pathogens. Fourie’s advice to growers is to maintain storage temperatures as close to 0°C as possible.

“Avoid temperature spikes, as some pathogens – especially Rhizopus, Penicillium and Botrytis – will multiply if the temperature increases to above 3°C or even 2°C.” He indicated that harvested grapes deteriorate more in one hour at 32°C than in one day at 4°C, or one week at 0°C.

Cracking down on berry cracking

Dr Justin Lashbrook, a researcher at Stellenbosch University’s Grape and Wine Research Institute, provided insights into new research on berry cracking. His presentation, “Mitigating Grape Berry Cracking Through Modulation of Berry Skin Elasticity”, highlighted the risk that cracking poses for the South African table grape export industry. “We know that there is an environmental connection (to cracking),” Lashbrook said.

“There is also a genetic link in that some cultivars are susceptible, and others aren’t. This means that there must be some genetic difference that leads to this difference in crackability. Regardless, when it occurs, the impact on harvests, costs and yields is massive, and storage life is also negatively impacted.”

Lashbrook is currently investigating how plant physiology causes this phenomenon. “The cuticular layer (the waxy layer that covers the berry surface) is adapted to undergo rapid expansion during fruit ripening, while maintaining its ability to waterproof the plant, block pathogens and prevent infection,” he explained. “Cracking occurs due to the elasticity of the fruit surface being unable to cope with the rapid expansion.”

Lashbrook says he suspects that the specific composition of the cuticular layer of grapes plays a role in this regard. Specific individual compounds are also giving rise to these different cracking qualities and are now under investigation.

Control measures to deal with pathogens range from vineyard practices before harvest (such as canopy management, good sanitation practices and fungicide applications) to practices after harvest (including packhouses).

According to Lashbrooke, early results indicate that peel elasticity does seem to play a more significant role than peel strength in crack resistance. “The big question is: Are we going to identify compounds in the cuticular layer that give rise to this elasticity? Results from other species (including tomatoes and cherries) suggest that it might be possible.”

“We also want to determine whether we can manipulate the formation of these compounds with growth regulators, thereby increasing crack resistance.”

Novel ways to predict browning in table grapes

Dr Andries Daniels, a researcher at ARC Infruitec-Nietvoorbij, explored the technology available to predict and classify browning in table grapes. Browning is a very complex biological process, he said. Berries usually move from healthy to unaffected, to brown over a gradient of time.

In his presentation, Daniels discussed a new method of near-infrared spectroscopy that is used to predict the browning of table grape bunches. “This cutting-edge technology is very versatile,” he said. “You can do measurements in reflectance transmission mode or absorption mode. This technology can also be coupled with machine vision, which can generate images while you are scanning the bunches.” This kind of visualisation can lead to real-time detection of browning defects and help classify bunches according to various aspects, including sugar and acidity levels.

Dr Juan Zoffili, a lecturer at the School of Agriculture and Forestry at the Pontificia Universidad Católica de Chile, offered comparisons of the dehydration potential of grape cultivars in his presentation. Zoffili explained that rachis browning (i.e. the browning of the main stem in a bunch that runs from the peduncle down through the cluster of grapes) is related to the dehydration-induced reduction in the total bunch weight of different cultivars. Cultivars like Thompson Seedless and Sweet Jubilee, for instance, take longer to develop brown rachides under high temperatures and 80% relative humidity than Sweet Celebration, Redglobe and Krissy do.

Under a low temperature of 0°C, Thompson Seedless and Red globe have the lowest score of rachis browning and Sweet Celebration and Krissy the highest, according to research. However, the rachides of Sweet Jubilee and Krissy deteriorate faster than those of other cultivars during shelf life.

Zoffili and his team are now working on a diagnostic index for browning, using non-destructive equipment. They also consider the water content of bunches at harvest and tissue damage in grapes associated with radiation. This is done to understand the susceptibility of different grape cultivars to chilling damage, as well as the ethylene sensitivity of new cultivars. In addition, the researchers are investigating the chemical compounds associated with tissue browning. And finally, they want to determine why red cultivars are usually more prone to developing rachis browning than other cultivars.