Many grape growers and winemakers dislike either rain or irrigation during the ripening phase of grapes because they fear that water will “dilute” their quality. This presentation will look at recent research that the Washington State University group has done, which highlights how grape berries can regulate their size and concertation by managing water input and output.

The beginning of the ripening phase (veraison) is characterized by an exponential increase in sugars and consequently water to meet the needs of the berry. This allows the grapes to ripen even under conditions of water stress. If the weather is warm and dry enough, the grapes can get rid of excess water by transpiring it from the skin; on the other hand, when the weather is humid, the excess water is distributed to the leaves.

The data used, supported by field trials, show that drip and sprinkler irrigation during the ripening stage do not cause quality dilution of the grapes; however, rain and sprinkler irrigation, or even very humid weather conditions, not only block evapotranspiration from the berries, but also lead to water ingress from the skin. Should the rains continue for a long time the berries will dilute their quality, but should the rains stop the berries will shrink, which tends to make them concentrate.

In conclusion, whether rain and sprinkler irrigation harm the quality of grapes depends on when they are harvested and whether they have sufficient time to lose excess water.

Short CV:
Markus Keller is a professor of viticulture at Washington State University in the northwestern part of the United States. He held a PhD from the Swiss Federal Institute of Technology in Zurich in natural sciences. He has been a researcher and professor of viticulture and vine physiology in Europe, Australia, North and South America. He had more than 100 scientific publications and authored the celebrated book “The Science of Grapevines – Anatomy and Physiology.”