Effect of Warming Rates on the Viability of Frozen Fungous Spores

Abstract

The successful application of ultra-low temperatures to the preservation of a broad range of living materials (1, 3, 4, 7, 9, 17, 19) has stimulated interest in the use of cryogenics as an alternative to freezedrying for the preservation of fungous cultures. Alternatives to freezedrying are needed, inasmuch as some fungi do not sporulate sufficiently to be handled by this method, and others simply fail to survive the freeze-dry process (5). Hwang (8) reported the successful use of cryogenic procedures for the preservation of a wide variety of fungous cultures, and, more recently, Wellman and Walden (21) and Hwang (unpublished) have found that the mycelium and hyphal tips of fungi will survive freezing and thawing. Successful preservation of the normally short-lived rust urediospores and pycniospores in liquid nitrogen has also been reported (1, 11), with the result that a collection of plant rusts has been initiated at the American Type Culture Collection (ATCC) (10). Based on the studies of Hwang (8, and unpublished), low-temperature preservation is being used increasingly at the ATCC for the longterm maintenance of fungi that do not sporulate or that otherwise fail to survive the freeze-dry process. In developing an expanded program for preservation of fungi by cryogenic methods, investigations on various aspects of the freezing procedure are needed to determine factors affecting the survival and recovery of fungi from the frozen state. Studies by Mazur (14, 16, 17, 18), Smith (19) and others (12, 13, 20) have shown that injury to biological systems can occur during both the freezing and thawing processes. Best results in preserving living material by freezing have generally been achieved when slow cooling at approximately 1 C per minute has been followed by rapid warming.1