SATI Beyond The Bunch 2nd Quarter 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