SATI Beyond The Bunch 2nd Quarter 2024

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SATI Research Booklet

Quarter 2

BEYOND

2 0 2 4

FOCUS ON: Climate manipulation

ADOBE FIREFLY

EUNICE AVENANT

Temporary plastic rain covers combined with overhead protective netting in Kamamas.

Creating a progressive , equitable and sustainable South African Table Grape Industry.

PROUDLY PRESENTED BY

WELCOME

About Beyond the Bunch This quarterly publication aims to serve as a science-based resource for the South African table grape industry. Each edition will feature an article exploring a topic or questions raised by industry, with links to related additional reading. Submit your topic or questions Producers and industry stakeholders are encouraged to suggest scientific or technical questions or topics of interest. We will strive to address as many of these as possible. Feature your brand SATI invites interested businesses and organisations to sponsor this publication. Sponsorship includes a banner and full page advert in the applicable edition. An advertorial option is available. Contact mecia@satgi.co.za to feature your brand. Email tarryn@satgi.co.za to submit your request.

BEYOND THE BUNCH • 3 • KWARTAAL 2 • 2024

PLASTIC PRACTICALITIES

The first in a two-part series on the basics of table-grape production under plastic.

Rain protection is the most common use of plastic covers in South African table-grape production. This article describes the practical aspects of rain covers and other applications. We start by contrasting earlier and later covering, and then look at the pros and cons of different systems. BY ANNA MOUTON

BEYOND THE BUNCH • 4 • QUARTER 2 • 2024

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Acknowledgements Thank you to the following people for assisting with these articles: • Eunice Avenant. SATI Training Manager and Extra-ordinary Lecturer in Viticulture in the Department of Viticulture and Oenology at Stellenbosch University. • Alwyn Dippenaar. Table-grape producer and co-owner of Dippenaar Choice Fruit in the Orange River Region. • Michele Melillo. Vice President of Global Technical Services at Sun World International. • Robert Oosthuizen. Agronomist at Sun World International. • Eurica Scholtz. Chair of the Berg River Table Grape Producers Association and SATI Director. • Rudi Slabbert. Farm manager at Karsten Farms on Kanoneiland in the Orange River Region. • Janéne Strydom. Table-grape researcher in the Viticulture Division at the ARC Infruitec Nietvoorbij.

BEYOND THE BUNCH • 5 • KWARTAAL 2 • 2024

EUNICE AVENANT

One application discussed below is manipulating the harvest window. Growers wishing to shift their harvest window should remember that plastic covers are part of a broader management strategy. Each grower must determine appropriate pruning and other interventions for their conditions and cultivars.

Covering earlier Deploying plastic covers during dormancy will raise vineyard temperatures and can advance bud break and subsequent ripening. Changing the harvest window may afford an advantage in specific markets or lighten the logistical challenge of harvesting many hectares of the same cultivar. Earlier covering gives more advancement than later covering. Italian researchers report that covering vineyards up to 50 days before the natural bud break will bring the harvest date forward by as much as 40 days. They consider this strategy most suitable for early-maturing and seedless cultivars. In South African studies, covering vineyards 35 or more days before natural bud break resulted in a 10–28-day advancement of the harvest date. In a commercial trial, plastic installed at bud break advanced the harvest by seven days. Heating calls for vineyards to be fully enclosed with plastic designed to trap heat. Chill requirements must be satisfied before grapevines are covered. Raising temperatures is only effective when low temperatures keep plants dormant – using plastics to advance bud break will have little effect in warm conditions. Be aware that too-high temperatures following bud break will partly reverse the advancing effect while possibly compromising berry size and colour. Growers should monitor temperatures and open the sides of enclosed structures as needed to avoid overheating. Plastic covers deployed within a week of bud break can boost growth and improve berry size and quality in white cultivars by creating softer growing conditions. Colour development in red and black cultivars can suffer when these are covered early. Early covering will help mitigate but not entirely address frost risk.

Covering later South African growers mainly use plastic covers to protect their table grapes from rain. Vines are usually covered at about véraison. Rain protection plastics have different radiometric properties than heat-trapping plastics – more on this in the next article. Rain covers can be elevated above or rest on the canopy and are usually open at the sides and over the work row to allow air circulation. Poor circulation contributes to dangerously high temperatures and humidity while potentially encouraging decay-causing microbes. Late covering can also prolong the harvest window of some cultivars by protecting bunches hanging on the vine. Italian researchers report this works best for late-ripening cultivars with large berries and strong skins. They note that maintaining berry hydration during prolonged hanging is critical and suggest applying a little extra irrigation and nutrition. Bunches must be monitored for disease. In a South African study, plastic protection of vines allowed grapes to be harvested 3–6 weeks later than usual. The covers didn’t delay ripening but protected grapes from rain and other adverse conditions so that the harvest could be delayed without increasing risks. Logistics partly determines the timing of covering. Growers with many hectares at risk may start covering earlier rather than face scrambling later in the season when labour is stretched thin. Growers with few hectares could let the weather forecast guide the decision to deploy covers. Keep in mind that shading delays sugar develop ment. This effect may be negligible if grapevines are covered late and light is carefully managed, but most growers can expect harvest delays of 2–3 days.

BEYOND THE BUNCH • 6 • QUARTER 2 • 2024

The two plastic systems Plastic covers fall into two broad categories: temporary and fixed. Temporary covers are plastic strips that rest directly on the vine canopies. Fixed covers are elevated above the vines on permanent support structures

similar to those used for protective nets. The two systems are compared in Table 1.

Table 1 The characteristics of different plastic cover systems summarised.

Temporary

Fixed

Support

Rests on vine canopy.

Post and wire structures.

Lifespan

Usually one year.

Plastic: 3–10 years. Structure: permanent.

Labour

Intensive to deploy and remove plastic.

Intensive to install structure. Less so to deploy and remove plastic.

Relative cost

Lower.

Higher.

Main risk

Excessive humidity and condensation.

Structural failure.

must compare plastics with similar radiometric and other properties when evaluating quotes. Labour and materials for support structures add to the cost of fixed covers – expect to pay at least as much and perhaps more than for shade-netting support structures. Saving on support structures will lead to, at best, sleepless nights and, at worst, collapsed covers. The plastic sheets of fixed covers last longer than temporary ones but must nonetheless be replaced regularly. Plastic lifespan is usually inversely related to price. Vine damage should be minimal with temporary covers or early deployment of fixed covers. Working with fixed covers later in the season becomes more challenging as the canopy expands. Northern Cape growers report problematical high humidity and condensation under temporary plastic strips. Western Cape growers could have better results, especially if they have drip rather than microsprinkler irrigation.

Temporary plastic strips are commonly used for rain protection. They can theoretically be rolled out quickly when needed, although this still requires sufficient labour to get the job done before it rains. In established vineyards, growers do not need to modify existing trellises for temporary plastic strips. Well-secured strips may be less prone to wind damage than elevated plastic covers. Fixed plastic covers are attached to permanent support structures that elevate them 50–100 centimetres above the canopy. Growers unfurl the plastic to cover their vines at some point before or during the season. They roll it up and store it attached to the supports after harvest. Support structures are best erected when new vineyards are established, but existing trellises can be modified for fixed covers. Plastic costs are a significant component of system costs and vary enormously depending on supplier and characteristics. This is another reason why growers

Nets and caps

However, fixed plastic covers are a possible substitute for nets when the primary goal is creating a softer growing environment. Some growers successfully protect their crops from rain by capping bunches individually, and caps may be a good alternative to temporary plastic strips for specific situations.

The interaction of protective netting and plastic covers depends on the grower’s aims. If fixed plastic covers and netting are too close together, they can restrict air circulation. Temporary covers are probably a better option under nets. Plastic covers will lessen the impact of hail but are not a replacement for hail nets in high-risk regions.

BEYOND THE BUNCH • 7 • KWARTAAL 2 • 2024

A PLASTICS PRIMER The second in a two-part series on the basics of table-grape production under plastic.

Extensive use of plastic covers as rain protection in the Puglia region of southern Italy.

BEYOND THE BUNCH • 8 • QUARTER 2 • 2024

Table-grape growers can use plastic covers for microclimate modification and harvest protection. Achieving success starts with understanding how plastic covers modify growing conditions, which is the topic of this article. The first article in the series discussed specific applications and systems. BY ANNA MOUTON

Light

flowering, and excessive shading early in the season can reduce bud fertility in the following season. Covering after véraison delays sugar development by roughly a week in all cultivars. Sugar development will be delayed less the later the plastic is deployed. Covering at véraison could still hold sugars back by 2–3 days. Covering white cultivars around bud break seems to allow the vines time to adjust and berries ripen in their usual window. Red and black cultivars should be covered when berries are fully or nearly fully coloured because shading reduces colour development. Most plastic covers block ultraviolet (280–320 nm wavelength) radiation to some extent. This protects the plastics but can have unintended consequences for the grapevines. Ultraviolet exposure boosts berry levels of antioxidants and flavours and can promote better red and black colour development. Growers should consider their regional and situational light environment when deciding on the amount and timing of shading for each cultivar.

Plastic covers have different radiometric properties – they change the amount and type of sunlight that reaches the grapevines. Manufacturers manipulate the plastic’s radiometric properties by tweaking the polymers and adding various chemicals. The formulations are usually proprietary, but the results shouldn’t be: growers need to know how much and what kind of light a plastic cover will transmit. The primary consideration is how much photo synthetically active (400–700 nm wavelength) radiation the plastic blocks. Some products can cause similar or even greater shading than white 20% shade netting. Combining protective netting and plastic covers will have an additive effect on light reduction. Shading generally increases vegetative growth, so rigorous canopy management is necessary for good light distribution. Insufficient light is detrimental to berry colour and quality and reproductive bud formation. Timing affects the impact of shading. Grapevines initiate the next season’s fruit buds during this season’s

EUNICE AVENANT

BEYOND THE BUNCH • 9 • KWARTAAL 2 • 2024

Temperature

Covering vines at bud break can partly mitigate frost risks and provide favourable conditions for early growth. The danger of overheating under plastic is most significant in very hot regions on very hot days – obviously. Temperatures in the high 40s have been recorded under plastic during trials in the Northern Cape, but the researchers observed no adverse effects on the grapevines. High relative humidity under the plastic likely helped the plants cope. Plastic strips resting directly on the canopy are associated with scorching, especially of leaves that are in contact with or compressed by the plastic. The damage is generally insufficient to affect grapevine performance or berry quality. Growers can avoid overheating – and even achieve cooler vineyards – by choosing the appropriate plastic and designing and managing the system to allow sufficient ventilation.

Not all plastic covers increase vineyard temperatures. The infrared (760–3 000 nm wavelength) component of sunlight causes heating, and plastics can promote more or less of a greenhouse effect depending on how much infrared radiation they transmit. Growers should obtain this information from their plastic supplier. Some plastics are designed to diffuse or scatter the incoming light. Diffuse light is associated with more efficient photosynthesis and lower leaf temperatures. The extent of warming also depends on climate and weather, with a relatively greater effect in cooler and sunnier conditions. Applying plastic covers during dormancy to raise vineyard temperatures is used to accelerate bud break and bring the harvest forward. Vines should be covered once their chill requirement has been satisfied. Covering vines before bud break in warm areas may increase blind buds and reduce berry mass and colour.

Temporary plastic rain covers over Brazilian vineyards.

BEYOND THE BUNCH • 10 • QUARTER 2 • 2024

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Water Plastic increases relative humidity by trapping moisture and restricting air movement. This can be good or bad. The upside of higher relative humidity is a softer growing environment with less water lost to evaporation and transpiration. Less irrigation is needed, and vines grow more vigorously. More vigorous growth will also be the downside of higher relative humidity unless growers adjust their irrigation and fertilisation. Table grape growers, in general, tend to err on the side of over-irrigation – doing this under plastic will promote canopy growth at the expense of reproductive growth and fruit yield and quality. Research in the Northern Cape found that a 20% reduction in irrigation of both open and covered vines had no detrimental effects on fruit yield or quality. Growers using plastic covers for rain protection should note that high relative humidity in itself can contribute to berry cracking. This is not usually a risk in typical summer-rainfall conditions with isolated showers interspersed with dry spells. It is more likely to occur in typical winter rainfall conditions – remember those? – of several days of continuous wet weather. However, in recent seasons, several days of continuous rain and wet weather have also occurred during the harvest period in the Orange River, Berg River, and Hex River Valley regions, contributing to berry cracking and other quality problems. High relative humidity under plastic can also lead to condensation that defeats the object of the covers. Problematic humidity is more likely under plastic covers that rest directly on the canopy and in structures without adequate ventilation. Raising plastic at least 80–100 millimetres above the canopy and having a gap above the work row improves air movement. Open sides likewise improve air movement and reduce humidity and temperature.

Other effects The modified microclimate under plastic can influence pests and diseases. Many diseases thrive in high relative humidity, but higher temperatures may put the brakes on conditions such as mildews. More research is needed to understand how plastic covers will affect crop protection. A positive of covers could be better spraying conditions. The still air and enclosed environment Vines also benefit from wind protection under covers, provided the installation can withstand wind. Learn more about plastic installation in the previous article, which discusses the specific applications of different plastic covers and provides practical tips on system design and management. should limit drift and improve deposition.

Sustainability South African table-grape growers are still experimenting with plastic covers, and many questions remain. In future, these will no doubt include the issue of sustainability as plastic covers add to the global plastic-waste burden – as do protective nets. Proprietary additives that alter radiometric properties and prolong plastic life may complicate or prevent recycling. At least one local plastics manufacturer has expressed an interest in recycling used covers, but recycling the many imported covers is likely to be challenging. Growers should consider how plastic covers will be assessed during environmental and sustainability audits to ensure continued market desirability for their fruit.

BEYOND THE BUNCH • 11 • KWARTAAL 2 • 2024