Observations on the influence of vine covering by means of a transparent plastic sheet on berry ripening and wine quality (Saint-Emilion, 1995 and 1996)

Given the climat of the Bordeaux area, major climatic accidents can affect both the yield and the quality of the crop. Spring frost can destroy the future harvest, as happened in 1977 and 1991. Heavy rain in September can compromise a promising grape potential, evaluated at the end of August, and cause major Botrytis problems, as happened in 1963, 1965, 1968, and, more recently, in 1993. In this paper we discuss how soil and vine covering can contribute to avoid these problems. Vine covering, by means of a transparent plastic sheet, was experimented with in 1995 and 1996, between the end of March and early May, to reduce frost harm. In September of the same years, at the end of the ripening period, we studied the influence of soil covering (mentionned "BS") as well as soil and vine covering (mentionned as « BT ») on water status of the vines, microclimate, berry ripening and wine quality. In 1996, 14 p. cent of the buds were frozen in the control plot. No frost damage was noted underneath the plastic cover. In the same year, when most of our observations were made, supranormal rainfall in August preceeded the second period of soil and vine covering, carried out on 27th August. Under these conditions, we did not mesure any difference in vine water status until the harvest on the three plots. Microclimate was warmer and drier on the covered plots, especially underneath the over-vine cover. On the covered plots, yields were higher. In 1996, on BT, the vines carried more bunches, the bunches carried more berries and berry weight was higher. The control vines were significantly more affected by Botrytis compared to BS ; BT showed almost no rot. Berries on the covered plots showed a tendancy of having more sugar and total phenolics, and less malic acid. Separate microvinifications were done with 50 kg of grapes from each plot. Wine from BT was preferred over BS. Wine from the control plot was the least appreciated.