Field testing of a genetically altered bacterium on potatoes in the Tulelake area will be delayed while residents and potato farmers are assured that the experiment is safe, University of California officials announced. The bacterium, called "ice minus," has been altered to prevent the formation of frost on the plants. Permission for the tests was given by the Environmental Protection Agency last week. James B. Kendrick Jr., UC vice president for agriculture and natural resources, said the university will do "whatever is necessary to address the concerns presented to us by the public. . . ."