Markus Keller opens Enoforum 2017

The fear of dilution: rain, irrigation and grape ripening

Many winegrowers and oenologists dislike neither rain nor irrigation during the ripening phase of grapes because they fear that water could ‘dilute’ their quality. This presentation will look at recent research that the Washington State University group has conducted, which highlights how grape berries can regulate their size and concordance by managing the inflow and outflow of water.

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

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

In conclusion, whether or not rain and sprinkling irrigation harm the quality of the 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 did his PhD at 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 has had more than 100 scientific publications and was author of the renowned book ‘The Science of Grapevines – Anatomy and Physiology’.