  We show how the threshold level of affine fusion, the fusion of
Wess-Zumino-Witten (WZW) conformal field theories, fits into the Schubert
calculus introduced by Gepner. The Pieri rule can be modified in a simple way
to include the threshold level, so that calculations may be done for all
(non-negative integer) levels at once. With the usual Giambelli formula, the
modified Pieri formula deforms the tensor product coefficients (and the fusion
coefficients) into what we call threshold polynomials. We compare them with the
q-deformed tensor product coefficients and fusion coefficients that are related
to q-deformed weight multiplicities. We also discuss the meaning of the
threshold level in the context of paths on graphs.
