  We show that principal bundles for a semisimple group on an arbitrary affine
curve over an algebraically closed field are trivial, provided the order of
$\pi_1$ of the group is invertible in the ground field, or if the curve has
semi-normal singularities. Several consequences and extensions of this result
(and method) are given. As an application, we realize conformal blocks bundles
on moduli stacks of stable curves as push forwards of line bundles on
(relative) moduli stacks of principal bundles on the universal curve.
