Sunscreen for Sauvignon blanc

Abstract

Kaolin is a chemical reflectant that has the potential to significantly reduce berry temperature, sunburn and other berry damages associated with cluster solar exposure while maintaining the concentration of important constituents such as acidity and sugar and maintaining (or even improving) yield.

Introduction

Sunburn of grapes is a physiological disorder resulting from a combination of excessive photosynthetically active radiation, ultraviolet radiation and elevated temperatures that can be exacerbated by other stress factors such as water deficit. The appearance of brown and necrotic spots on the epidermis of grapes (which can further develop into the complete desiccation of the berries), severely affects the commercial value and yield of the cultivated fruit. Depending on the severity of the damage, wine grapes can be downgraded in quality due to sunburn damage, resulting in economic loss.

Many viticultural management practices directly affect fruit sunlight exposure and, therefore, sunburn incidence. Practices such as canopy management, row orientation, trellis and training systems, and soil and irrigation management strategies can all affect the degree of sunburn damage during extreme climatic conditions. However, sunburn protective applications are increasingly attracting attention in research and commercial settings, and international grape growers are getting creative in protecting their crops from the elements during extreme weather conditions. Some of these newer protective measures include the use of netting, antitranspirants, hydrocooling and particle film-forming products.

Kaolin – a chemical light reflectant

One of the particle film-forming particle technologies is an inert chemical reflectant named kaolin (aluminium phyllosilicate, Al₂Si₂O₅(OH)₄, nanoparticles), a white clay that can reflect ultraviolet and infrared radiation. When sprayed over the canopy, the light and temperature conditions around the leaves and bunches change, directly affecting the vine’s gas exchange rate. Its ability to prevent sunburn has been successfully tested in several fruit crops.

Two research groups investigated the effect of kaolin application on Sauvignon blanc vines and its impact on the vine physiology and grapes produced. Indeed, the research group from Uruguay confirmed the protective effect of kaolin when applied (during the post-vériason to harvest period) to defoliated Sauvignon blanc vines:¹

• The maximum berry temperature of the kaolin-treated vines was 7.2°C cooler compared to the maximum berry temperature of the defoliated vines with no kaolin spray. The temperature of the treated vines was comparable to the control treatment (no defoliation, no kaolin application).
• The kaolin application reduced cluster sunburn incidence by more than 50% when compared to the non-sprayed defoliated vines.

Another study investigated the effect of kaolin application (at bunch closure and vériason) on Sauvignon blanc vines (not defoliated) and grapes grown in the northeastern part of Italy.² Some of the main findings were as follows:

• The kaolin application resulted in improved yield performance compared to the defoliated vines (no kaolin application) and the control vines (no defoliation, no kaolin application). The average bunch weight of the kaolin-treated vines increased by approximately 10% when compared to the control.
• Kaolin application resulted in an increased final titratable acidity concentration in the berries (6.7 g/L) when compared to the defoliation treatment (no kaolin application) (5.8 g/L) and was similar to the control treatment (6.3 g/L).
• Similar to the titratable acidity, kaolin application resulted in berries with a higher soluble solids content at harvest (by approximately 0.65°Brix) when compared to the berries obtained from the defoliated vines without kaolin application and was comparable to the control treatment. However, these results were not statistically significant.
• No significant differences in pH, total phenolics and antioxidant potential were reported.

The research group from Uruguay also measured the 3-isobutyl-2-methoxypyrazine (IBMP) concentration in the berries.¹ IBMP is an odorous compound responsible for the green pepper, grassy and herbaceous aromas. It can be a positive quality factor when it is not dominant, but balanced, complemented and supported by other aroma compounds. As expected, there was a significant difference between the IBMP concentration of berries obtained from the control vines (no defoliation, no spray) and the berries obtained from the defoliated vines, with the control vines delivering berries with a higher IBMP concentration. Interestingly, despite the difference in maximum berry temperature, there was no significant difference in the IBMP concentration of the berries obtained from the defoliated vines, either with or without the kaolin application. The group argued that the results could be due to the timing of the leaf removal, which was performed post-vériason as opposed to pre-vériason, which might have a more significant effect on berry methoxypyrazine concentration. However, this specific finding should be considered and tested further to ascertain this important effect.

Conclusion

Kaolin has the potential to significantly reduce berry temperature, sunburn and other berry damages associated with cluster solar exposure while maintaining the concentration of essential constituents such as acidity and sugar and maintaining (or even improving) yield. The application of kaolin could mitigate damage from heatwaves and suboptimal sunlight intensity or help adapt established vineyards to changing climatic conditions. Kaolin application also enables the producer to perform canopy management strategies such as leaf removal while still protecting the grapes from excessive photodamages.

The application of kaolin might also be helpful after sunburn has occurred. The application can prevent the spread of sunburn symptoms, especially if adverse meteorological conditions persist. The application might prevent damage to berries in the cluster interior that are suddenly exposed to sunlight by the shrivelling of exterior berries and might also prevent damage to the rachis.³

The effectivity of kaolin application appears to be strongly related to other cultivation and environmental factors and needs to be investigated further, especially with the unique local climate. Moreover, the variation in the concentration of the active ingredient and the number and timing of applications might be important aspects for further investigation. The economic effect of re-applications due to the washing-off effect in higher rainfall regions should also be considered.²

References

1. Ferrari, V., Disegna, E., Dellacassa, E. & Coniberti, A., 2017. Influence of timing and intensity of fruit zone leaf removal and kaolin applications on bunch rot control and quality improvement of Sauvignon blanc grapes, and wines, in a temperate humid climate. Scientia Horticulturae 223, 62 – 71. https://doi.org/10.1016/j.scienta.2017.05.034.
2. Andreotti, C., Benyahia, F., Petrillo, M., Lucchetta, V., Volta, B., Cameron, K., Targetti, G., Tagliavini, M. & Zanotelli, D., 2024. Comparing defoliation and canopy sprays to delay ripening of Sauvignon blanc grapes. Scientia Horticulturae 326, 112736. https://doi.org/10.1016/J.SCIENTA.2023.112736.
3. Gambetta, J.M., Holzapfel, B.P., Stoll, M. & Friedel, M., 2021. Sunburn in grapes: A review. Frontiers in Plant Science https://doi.org/10.3389/fpls.2020.604691.

For further information, contact Carien Coetzee at carien@basicwine.co.za.

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