The military said in Manila that it has uncovered a plot by Communist guerrillas in the Philippines to assassinate retired U.S. Maj. Gen. John K. Singlaub. Col. Oscar Florendo, military spokesman, said the plot was uncovered after decoding computer disks seized during the capture of 20 leaders of the outlawed Communist Party. Florendo said the plot also targeted Gen. Renato de Villa, chief of staff, and Brig. Gen. Honesto Isleta, former military spokesman. Singlaub visited Manila several times last year and was accused of helping to organize vigilantes to fight Communist rebels. He denied taking part in any anti-rebel efforts. He is currently in the United States.