Beta-mannosyltransferase involved in cell wall biosynthesis. Required for the addition of beta-mannose to the acid-labile fraction of cell wall phosphopeptidomannan. Regulated on yeast-hypha and white-opaque switches, and repressed in biofilm. Impairs the elongation of beta-mannose chains on the acid-labile fraction of cell wall phosphopeptidomannan. Belongs to the BMT family.