SATI Beyond The Bunch 2nd Quarter 2024

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