  Using a loop formulation approach of QCD$_2$, we study the potential between
two heavy quarks in the presence of adjoint scalar fields, and demonstrate how
't Hooft's planar rule is manifested in this formulation. Based on some
physical assumptions, we argue that large adjoint loops ``confined'' inside an
external fundamental one give a Casimir type contribution to the potential
energy, while the small loops only renormalize the string tension. We also
extend the results to the case of massive adjoint fields.
