Just like human skin, grape skin also burns when exposed to too much direct sunlight. So while sunlight is key for ripening of the grapes, it easily damages the skin of the grapes.
On grape clusters, sunburn looks like a white, brown or yellow blotch on the side of the cluster that is directly exposed to sunlight. This happens more often on sides of grapevines that face south or west.
The blotch may be soft at first. But after a period of time the affected area dries up and creates a berry that is hard on one side and partially mummified. Sunburned grapes do not ripen as well as normal grapes. Small amounts may not impact wine quality, but severely sunburned clusters should be dropped.
We work very hard to combat this by working with the canopy of the vines from early on in the growing season in order to provide shade for the grapes clusters and protect their skin.