Évolution de la surmaturation du cépage Chenin dans différents terroirs des Coteaux du Layon en relation avec les variables agroviticoles

The French “Coteaux du Layon” Appellation of Origin has built its Jarne on the production of sweet white wines. A network of experimental plots, based on the “terroir” concept, was established in 1990; it allows for the follow-up of the overripening behaviour of the grapes in relation with the agroviticultural parameters. The studied terroirs show a distinctive behaviour; they can be classified in two groups. One favours a precocious and important development of Botrytis cinerea, which can give way to the noble rot with more or less intensity according to the vintage; sugar concentration never reaches high levels and deviations towards the grey rot and other fungal and bacterial infections are frequent. On the other the overripening process leads to a late and less important settlement of Botrytis which moves towards the noble rot form, with passerillage; their relative proportions vary according to the type of terroir and the vintage. On the first terroirs, the Chenin variety has a rather Late cycle and an important vigour. On the latter, the plant cycle is always more precocious and the vigour less important. These parameters are related to the degree of evolution of soils on a given geological substratum and the abundance and regularity of water supply. The differences within each group can be explained by local climatic variations such as exposition, sunshine and the frequency of morning mists.