After his retirement in 1966, Harrison began studying the mushroom-producing species of the Agaricomycotina more avidly. Collections forays throughout Nova Scotia, in the Great Smoky Mountains, New Mexico, the Upper Peninsula of Michigan, and the Pacific Northwest led to him describing several new species in the genera Steccherinum, and tooth fungi in Hydnellum, Sarcodon, Phellodon. Harrison died in Kentville, Nova Scotia, in 1991. The Department of Biology at Acadia University has a mycology laboratory named in his honour.