Every fibration with totally path-disconnected fibers has the unique path lifting property (2.2.5 of E.H. Spanier, Algebraic Topology, McGraw-Hill, New York, 1966) implying that $p_{\ast}$ is injective. However, it seems to me that $p_{\ast}$ is not likely to be surjective unless you impose more conditions on the spaces $Y$, $\widetilde{Y}$, or the map $p$.