Humagne Rouge: Moderate Water Stress Improves Wine Quality

Tests carried out by Agroscope in the Valais region reveal that the Humagne Rouge grape variety yields better wines and is less susceptible to bunch shrivel when grapevines are not irrigated.

Sensible water use within the context of global warming, resource conservation and changing farming practices poses a major challenge in Swiss vineyards, particularly in the Valais.  On hillsides with a southern exposure, with their pebbly, pervious soils, winegrowers sometimes resort to irrigation. Water scarcity can trigger stress reactions in plants and impair wine quality.

Water scarcity changes grape composition…

On the other hand, we know that moderate, gradual water constraint over the course of the season encourages the production of grapes that are rich in sugars, anthocyanins and polyphenols. These conditions are ideal for the creation of superb-quality red wines for laying down.  Irrigation practices must be formulated in association with the choice of plant material (grape variety/rootstock pairing), the nature of the soil and its water reserves, soil maintenance, and other cultivation techniques such as management system and fertilisation, where applicable.

…and reduces the risk of bunch shrivel

Furthermore, studies have shown that grapevine water balance strongly influences the susceptibility of the grape varieties to a physiological disorder known as bunch shrivel, which disrupts the berry-ripening process and leads to the appearance of highly acidic bunches with a low sugar content. The Humagne Rouge grape variety is particularly vulnerable to this phenomenon.

An irrigation trial was conducted from 2010 to 2016 in Leytron in the central Valais region, where conditions are relatively dry, to study the effect of vine water balance on grape ripening and Humagne Rouge wine quality. Some of the grapevines were drip-irrigated from flowering to veraison (in general, from 15 June to 15 August); another group was left unirrigated, and a third group was deliberately subjected to significant water stress by placing a waterproof, non-reflective tarpaulin on the ground to eliminate rainwater from the end of April up until the harvests.

Better wines, healthy vines, stable yields

The irrigation trial confirmed the positive effect of moderate water constraint during Humagne Rouge grape ripening. This effect was associated with:

–           a better accumulation of sugars in the berries

–           a lower overall acidity content and malic acid content of the berries

–           more-deeply-coloured, polyphenol-rich wines

–           wines with softer, more unctuous and more-structured tannins

–           wines that are more highy rated at tastings

Yield components (bud fertility, berry weight) were not influenced greatly by the moderate water restriction. Only those grapevines subjected to significant water stress due to the ground tarpaulin showed a slightly reduced yield. Moreover, all unirrigated vines were far less affected by bunch shrivel, as anticipated.