L'étude biométrique d'un essai destine à la mise au point d'une méthode d'appreciation de la valeur de produits fongicides (anti-mildiou), montre que la technique de brumisation permet de faire des essais valides même dans des conditions climatiques peu propices au mildiou car on obtient une infection sévère et uniforme, et l'ordre de classement des produits est sensiblement Je meme qu'en conditions naturelles.Les plantes ayant subi des destructions du feuillage très diverses suivant les traitements on été étudiées pendant trois ans. Cette étude démontre l'existence du cycle de trois ans: constitution de réserves, initiation florale, production de grappes déjà postulé par ailleurs.Observations on the cycle determining the yield of the grapevine made while perfecting a method of artificial infection by mildew (Plasmopara viticola) in order to evaluate fungicides in the fieldSpraying a water mist ("brumisation") during the summer months results in a strong and even downy mildew infection on one year old rooted vine cuttings grown in the field. This makes comparative trials of fungicides possible even in unfavourable (dry) years.Such a trial, involving four levels (spray formulations) of the fungicides factor, two levels (with or without) of the mist factor, with five replications in plots of six plants, resulted in a set of plants having suffered various degrees of foliage destruction. Measuring these plants during three successive years, starting with the differential treatment year, provided the basis for checking an hypothesis on the determination of yield in the grapevine.Progressive analysis of variance of regressions (WooLF 1951) was used in the interpretation of results.Vigour in the second year was negatively correlated with foliar damage in the first year. In turn, number of bunches in the third year was correlated with vigour in the second year, hence with foliar damage in the first year.This lends support to the hypothesis that, in the grapevine, there is a three year cycle leading to the harvested clusters.On an adult plant, three cycles are simultaneously at work at every moment. These cycles are competing for energy fixed by photosynthesls; hormonal correlations that may be correlated with energy (carbohydrates) storage, for instance through its influence on root system volume, are also at play.The implications of these results, especially for experimental work with the grapevine, are discussed.