The String Calculation of QCD Wilson Loops on Arbitrary Surfaces
  Compact string expressions are found for non-intersecting Wilson loops in SU(N) Yang-Mills theory on any surface (orientable or nonorientable) as a weighted sum over covers of the surface. All terms from the coupled chiral sectors of the 1/N expansion of the Wilson loop expectation values are included. 
Stephen G. Naculich and Harold A. Riggs