  The double-exchange model for the Mn oxides with orbital degeneracy is
studied with including on-site Coulomb repulsion, Jahn-Teller (J-T) coupling
and doping-induced disorder. In the strong interaction limit, it is mapped onto
a single-band Anderson model, in which all scattering mechanisms can be treated
on an equal footing. A sharp rise in the mean square fluctuation of lattice
distortions is found near the Curie temperature $T_c$, in agreement with
experiments. We show that the spin and J-T disorders lead to a metal-insulator
transition (MIT) only at low carrier density. The MIT observed in samples with
$0.2\leq x<0.5$ can be explained by further including the disorder effect of
cation size mismatch.
