Grape quality FACTORS

Heat, drought stress and rainfall

In his keynote address, Dr Markus Keller (pictured) spoke about the consequences of heat, drought stress, and pre-harvest rainfall, and how these can be mitigated. Keller is the Chateau Ste. Michelle distinguished professor of viticulture in the Department of Horticulture at Washington State University.

His presentation was well received by participants, who showed a keen interest in factors such as irrigation techniques, the use of plastic sheets as protective measures, and how to avoid berry splitting.

Keller emphasised that grape yields are compromised by drought stress during bud break and the
growing season.

"If there isn't enough soil moisture in the ground at these times, the vine is unable to initiate root pressure," he said. "It cannot dry its own hydraulic system, so it cannot push out air bubbles from this system. This means the roots cannot really start growing. Erratic bud break, slow shoot growth, cluster abortion, and poor fruit set follow, causing yield loss."

Keller said it is vital that producers check the soil moisture in their vineyards and irrigate during bud break. Water stress during bloom as well as before and after veraison (the onset of ripening) affect yield negatively.

"If you want to maximise canopy development early in the season, you need roughly 3 – 4% soil moisture below field capacity," he said. "Don't give the vine too much water. The grapes do not like wet feet or wet roots."

Water and heat stress both cause berry shrinkage, which also affects yield. After veraison, certain irrigation practices might prevent further shrinkage, but it cannot reverse it.

"One thing to keep in mind is that grape berries are designed to minimise water loss. They are not like leaves; their transpiration rate is about 100 times or so lower than the leaf transpiration rate.

"The berry doesn't want to transpire water; it wants to move the water in so it can grow, which means it cannot control the temperature variable. They (the berries) overheat very easily. This means that they can get sunburned when overexposed to the sun. Your canopy needs to provide some level of shade to keep the berries happy."

The drier and warmer the surrounding air gets, the more water evaporates. "But as the air gets cooler and more humid at night, less water evaporates, meaning that berry transpiration increases and decreases on a daily basis."

"It's very much driven by the temperature and humidity of the atmosphere of the surrounding areas," he explained. "This also means that if a berry is exposed to drought and heat stress, it can shrink. So, when we irrigate, there is some berry growth, but then the berry size decreases again in a stepwise manner."

"Excess water before veraison causes berry expansion but not splitting," Keller said. "Conversely, excess water after veraison does not lead to expansion but does cause splitting."

"Berries only grow at night. They usually shrink a little during the day, as they lose water to transpiration. If you supply water before veraison, the berry does increase in size again. If you do the same thing after veraison, you cannot compensate for any previous loss in berry size. You can just stop it from shrinking even more."

Split happens

Berry splitting resistance drops during initial berry softening. "Drought stress before veraison is linked to berries having a higher susceptibility to splitting," Keller said. "Dehydrated berries do not split. Therefore, delay harvesting after rain."

Too much water late in the season due to rainfall, sprinkler irrigation, or high relative humidity – either before or after harvest – causes low berry transpiration. This, in turn, can affect splitting, Keller emphasised. "The situation gets even worse if you get surface water absorption on these berries."

Skin ("cuticle") thickness is not related to splitting susceptibility, he pointed out. "Splitting usually originates from existing micro-cracks."

"A thick berry skin does not prevent it from splitting. Also, if you have berries splitting, you likely also have botrytis," he says. "This means we must control botrytis early, even during flowering."

Yield and berry size are determined before veraison, Keller continued. "Pre-veraison, irrigate more to increase berry size," he said. "After veraison, irrigate enough to limit berry shrinkage, but not too much to delay ripening and prevent shoot growth, which will affect yield and quality."

Keller advised producers who want to prevent berry splitting to avoid water stress before veraison. He also urged producers to consider harvesting during the day when the berries lose a certain amount of water, seeing as they get some of it back during the night.

"Treat rainfall like irrigation events. Consider the amount, duration, and level when making irrigation decisions shortly after the rain. Give the vine a little time to dry out. Remember, the berry slowly hydrates over time."

Curbing pathogen-related decay in table grapes

In his presentation, Dr Johan Fourie discussed the role of weather conditions, chemicals, and in-vineyard sanitation in managing postharvest grape quality. Fourie is a research fellow and former managing director of ExperiCo Agri-Research Solutions.

While decay symptoms in table grapes typically develop during storage only, pathogen infections may already start pre-harvest during flowering or the growth and development phase. This is especially likely under disease-conducive conditions.

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 those in packhouses).

He encouraged producers to consider disease cycles and the level of decay present when devising risk management strategies. "If decay increases, select venues with lower disease levels for harvest and
packing at that time."

Producers should also avoid harvesting affected and healthy grapes at the same time, he advised.

Moreover, the cold chain must be managed carefully during and after forced-air cooling, during shipment and distribution, and at the point of retail. In addition, storage temperatures must be managed to avoid the spread of pathogens.

Disease spread and fungicide sensitivity

In his presentation, Prof Francois Halleen shared new insights into the epidemiology of powdery mildew and its implications for the management of this fungal problem in grapes. Halleen is a specialist researcher at the Agricultural Research Council (ARC).

According to him, as climate change and other factors alter the production environment, table-grape producers will have to pay more attention to certain diseases and their spread.

Halleen also emphasised the need to evaluate the sensitivity and efficacy of the fungicides most widely used to deal with powdery mildew. In addition, he calls on researchers to urgently reassess the understood source and transfer of the primary inoculum in Western Cape vineyards.