Submitted to: Good Fruit Grower
Publication Type: Trade Journal
Publication Acceptance Date: 2/22/2005
Publication Date: 4/1/2005
Citation: Tarara, J.M., Spayd, S.D. 2005. Tackling 'sunburn' in red wine grapes through temperature and sunlight exposure. The Good Fruit Grower. 56(3)40-41.
Technical Abstract: As vineyard managers adopted regulated deficit irrigation, more "sunburn" or scald was observed on the grapes. Reducing the vine's foliage through better irrigation management may have promoted an unintended consequence: excessive exposure of fruit to sunlight. Sunlight influenced the composition of Merlot wine grapes in two ways: direct solar radiation and the resultant fruit temperature. Although morning and afternoon sun are equally intense, berries sunlit in the afternoon will be subjected to higher absolute temperatures, usually around 4 p.m., because ambient temperatures are the highest of the day. Shaded berries remain near air temperature all day. Only west-facing fruit bore symptoms of sunburn. Sun exposure and fruit temperature did not affect Brix, while TA was inversely related to the overall temperatures to which the fruit were subjected: the greater the heat, the lower the acidity. Lower concentrations of anthocyanins in west-facing, sun-exposed berries probably were due to excessive fruit temperature around mid-afternoon. Pigments formed more slowly, degraded more quickly, or both. Temperature had no measurable effect on flavonols, but light increased total concentrations of both flavonols and anthocyanins. Chilling sunlit clusters, (eliminating excessive heat but allowing sun exposure) resulted in high concentrations of anthocyanins and intense color in berry macerate. Some sunlight is needed to maximize anthocyanin synthesis and to balance other wine-related compounds like acids and flavonols. Partial shade might be provided in a vertical canopy by less aggressive shoot positioning on the west side of the canopy. Leaf stripping should be minimized in high light environments subjected to warm temperatures during the ripening period.