Overhead nets and plastic in SA table-grape production systems

Preliminary findings

• All plastic-cover treatments (T3-T6) produced significantly higher yields than the uncovered control.
• T4 (overhead plastic applied after fruit set) achieved the highest export yield and showed superior berry firmness and pedicel condition after cold storage.
• T4, T5 and T6 emerged as the most promising treatments for further investigation.
• T3 is not recommended, as it resulted in the highest cull percentage and lowest export percentage due to brown physical markings on the fruit.

Microclimate data

All cover systems transmitted sufficient light for photosynthesis. Wind speed was reduced under cover systems, while temperatures remained within acceptable ranges for grapevine physiological processes. Conditions favourable for berry cracking were not recorded during ripening and harvest.

Project 2: Rain protection covers for harvest timing manipulation (Hex River Valley)

The vineyard was established in 2019 at De Hoop in the Hex River Valley, using Sugrathirty- five grafted onto Ramsey rootstock. The study investigates protection systems (overhead plastics, nets and bunch bags) to protect grapes from rain and to manipulate harvest timing, particularly by delaying harvest while maintaining fruit quality. The experimental site comprises two complementary trials: Trial 1, which evaluates a range of overhead cover protection systems applied at different developmental stages, and Trial 2, which assesses the potential of selected cover treatments and bunch bags to extend harvest timing through staggered harvesting dates.

Key findings

• Plastic-covered treatments generally showed slower ripening than the control.
• In Trial 1, treatments T3, T4 and T5 demonstrated potential to delay harvest.
• In Trial 2, harvest date could be delayed by approximately two to four weeks with use of covering treatments.
• Plastic covers reduced decay and berry cracking compared with uncovered vines.
• Plastic covers and paper bags did not, however, completely prevent postharvest berry cracking.

Microclimate data

All systems maintained adequate light levels for photosynthesis and temperatures remained below critical thresholds. High humidity and dew-point conditions during ripening and harvest created favourable conditions for berry cracking and microcrack development.

Project 3: Practical industry evaluation of plastic covering materials (commercial validation trials)

Unlike Projects 1 and 2, Project 3 did not involve replicated treatment comparisons. Instead, commercial vineyards across production regions evaluated different plastic-cover products under practical farming conditions. As noted by the project team, these should be regarded as evaluation scenarios rather than treatments.

The different cover materials were evaluated for one or more of the following purposes:

• rain protection
• microclimate modification
• harvest-delay potential.

Key findings

• Advanced ripening under covers was recorded at several commercial sites, including Blouputs, Groblersdal, Newgro and Denau.
• Microcracks and berry cracking occurred under both covered and uncovered conditions during periods of high humidity.
• Similar observations were recorded in Project 2, suggesting that plastic covers alone cannot eliminate cracking when environmental conditions are highly conducive to its development.

Overall recommendation

Preliminary results from the first season indicate that plastic-cover systems have significant potential to improve table-grape production and quality, while also offering opportunities to manipulate harvest timing. In Project 1, several cover treatments increased yield, export pack-out and fruit condition, while Project 2 demonstrated the potential to delay harvest by up to several weeks and reduce decay and berry cracking.

Commercial evaluations in Project 3 provide evidence that plastic covers can modify vineyard microclimate and influence ripening patterns under commercial conditions. However, berry cracking and microcracks were still observed under both covered and uncovered conditions. This highlights the important role of environmental factors, particularly humidity, in determining outcomes. As these findings are based on a single season, further evaluation is required before definitive recommendations can be made.