We propose a density-functional theory for the study of the phase behavior of discotic molecules interacting via a Gay-Berne potential. The attractions are treated perturbatively in a van der Waals-type manner while the virial series of the repulsions is resummed approximately with the inclusion of the exact second-order Onsager result. On this basis we have studied the isotropic (I)-nematic (N)-columnar (Co) phase behavior for various aspect ratios of the discotic molecules. For small aspect ratios (i.e., large nonsphericities) the phase diagram is found to involve a I-N-Co triple point where the two I-N and N-Co first-order transitions are transformed into a direct I-Co first-order transition. For larger aspect ratios the domain where the N-phase is stable shrinks and disappears above a critical value of the aspect ratio. When compared to the available simulation data it is found that the theory underestimates this critical aspect ratio.