This paper proposes PLPD, a novel percolation-based probabilistic reliable dissemination protocol for information dissemination among a group of nodes in a MANET. Being different from other probabilistic reliable dissemination protocols, PLPD is aware of the network topology and directed dissemination, which doesn't require flooding the network with control messages to manage group members. In PLPD, each group member maintains only a partial view of other members in its neighborhood and disseminates data messages with probability p to a subset of the view. The dissemination process of the PLPD protocol is modeled with the percolation theory, and it is proved that PLPD can achieve a probability close to 1 for all group members to receive every message, if p is greater than a certain critical threshold. The simulation results show that the PLPD protocol effectively reduces the network load while providing high reliability, and scales well to large system sizes.