The blank column is displayed only if the control's TreeListOptionsView.AutoWidth option is disabled and the total width of all the column headers is less than the Tree List's width.
The table below lists the main properties which affect the element's appearance:
The TreeListAppearanceCollection.HeaderPanel property.
Note: the appearance settings which specify the background and border colors are ignored if a Tree List is painted in WindowsXP or Office2003 style.
Custom Draw Event
The TreeList.CustomDrawColumnHeader event.