Sunburn damage manifests as a browning of the berry skin, and depending on the severity of the damage can lead to cracking and shrivelling of the grapes which reduces the visual and aromatic quality of the grapes causing economic losses to grape and wine producers as a result losses in yield and wine quality. In areas where leaf removal is a common practice, better understanding of the best moment to conduct this practice could help mitigate this problem. During the 2018-19 season the effect of leaf removal timing on sunburn incidence and on the accumulation of photoprotective compounds was evaluated on Chardonnay grapes from 2 vineyards located at different altitudes in the Orange region. Leaf removal led to higher final concentrations of all photoprotective compounds when compared to the non-defoliated control. Marked differences were observed in the accumulation patterns between berries defoliated early (end of flowering) and those defoliated later (véraison), with earlier accumulation, and thus better acclimation in those that were exposed early. Ultimately, sunburn damage was higher in grapes defoliated at véraison. There were no significant differences between yield, total soluble sugars, pH, titratable acidity and YAN between treatments within each vineyard.
Leaf removal is a necessary practice in cool climate vineyards to decrease disease pressure. However, this practice can also overexpose berries and lead to sunburn, particularly in the event of heatwaves. Sunburn damage is linked to the accumulation of melanin-like pigments and degradation of chlorophyll. When exposed to sunlight, particularly UV-A and UV-B radiation, grapes also produce flavonol compounds (i.e. quercetin glycosides) and carotenoids which have been shown to be involved in UV screening and act as photoprotective compounds.
This research project evaluated the impact of leaf removal timing (no defoliation, defoliation after the end of flowering and defoliation at véraison) on the severity of sunburn symptoms in Chardonnay grapes from two vineyards located at different altitudes in the Orange region. The accumulation of photoprotective compounds was also assessed. No significant differences were observed in relation to pH, titratable acidity, total soluble solids and YAN between treatments within each vineyard. Results showed a higher incidence of sunburn damage on berries that were defoliated after véraison when compared to those where leaf-removal occurred earlier in the season, after the end of flowering. Higher final concentrations of both flavonoids and carotenoids were observed in berries that had been exposed to light (through both early and late defoliation) than in the non-defoliated controls. Although no significant differences were found in the concentration of most of the evaluated photoprotective compounds at harvest between the early and late defoliated treatments, marked differences were observed in the pattern of accumulation – concentration of these compounds rose rapidly in berries defoliated at flowering and remained high throughout the rest of the season - which appear to have led to better acclimation and a lower degree of sunburn in the berries that were exposed at the end of flowering. 2019 was a particularly harsh season which broke most of the national records for the month of January. When ripening occurred mainly during this month (lower vineyard), higher sunburn damage was observed. An app (RotBot) was also tested to determine whether image analysis could be used as an objective tool to evaluate sunburn damage. Results showed that image analysis is a valuable tool that could be used by producers after further calibration of the algorithm. In areas at risk of sunburn where defoliation is a common practice, the results suggest that performing leaf-removal as soon as the end of flowering would help protect berries from sunburn without affecting yield and sugar accumulation.