Remove all of the clusters on grapes the year of planting.  The second year, you may leave one or two clusters on more vigorous vines.  The third year after planting, if the vine has filled its allotted space, you may leave the clusters on the vine.

Large clustered varieties of grape such as Reliance, Seyval blanc, Chambourcin, or Vidal blanc will benefit from annual cluster thinning.  This is where you remove extra clusters on the shoot and leave only one per shoot.  This is done after berry set when the small grape berries are shot size (see photo left).