You need to set the LabelPosition on the ColumnSet to None. How you do this depends on whether or not the tree is bound at run-time or design-time and when the column set gets created. 

If you are binding the tree at run-time, then you could do something like this: 

        private void ultraTree1_ColumnSetGenerated(object sender, Infragistics.Win.UltraWinTree.ColumnSetGeneratedEventArgs e)
        {
            e.ColumnSet.LabelPosition = Infragistics.Win.UltraWinTree.NodeLayoutLabelPosition.None;
        }