426 PHYSICAL AGENTS 



assayed after thawing the material, and, as shown later (Figure 2), the 

 rate of subzero warming is critical. 



Exposure to very low temperatures, e.g., that of liquid air, does not 

 completely kill mycelium, fruiting bodies, or dry spores of most fungi 

 (8, 16, 79, 89). The mycelium of some fungi does not survive this 

 treatment (89), and wet conidia of Neurospora crassa are killed almost 

 immediately at -170° to -190°C (40). 



Exposure to moderately low temperatures — from to — 40°C — is of 

 immediate ecological interest and has frequently been investigated. 

 The results, however, defy any neat generalization; either fungi differ 

 very widely in their tolerance or, as suggested above, methods of investi- 

 gation are so different that comparison is not legitimate. Mycelium of 

 Phycomyces nitens and Botrytis cinerea is killed by exposure to —15 to 

 —22° (6), and neither mycelium nor sclerotia of Phymatotrichum om- 

 nivorum survives 24 hours at —13° (39). But mycelium of Phacidium 

 infestans and Collybia velutipes is still viable after 138 days at —21° 

 (114), and fruit bodies of Schizophyllum commune withstand exposure 

 to -15 to -40° (17). 



Similar differences are reported in the survival of spores at these 

 moderately low temperatures. Uredospores of the rust fungi are usu- 

 ally found to survive no more than 2 months or so at —15° to —40° 

 (22, 42, 104, 149), but at least some uredospores of Melampsora lini are 

 still viable after 1295 days at —10° (41), and uredospores of Uromyces 

 phaseoli live as long as 2 years at —20° (59). Both spore forms of 

 Endoconidiophora fagacearum are germinable after 83 days at —10° 

 (24). 



From these reports and from the partial list compiled by Luyet and 

 Gehenio (91), it appears that spores are in general less damaged by 

 low temperature than mycelium, dry spores less than wet. 



Most of the reported experiments on the true fungi are probably 

 examples of relatively slow freezing, in which the primary injurious 

 factor is dehydration; crystals of ice form extracellularly and withdraw 

 water from the cell (105). Damage to dry cells is less than to wet (14, 

 40), as would be expected. Furthermore, it was early found that 

 solutes protect the mycelium against freezing injury (6); hydrogen 

 bonding substances — sugars, glycols, etc. — essentially lower the amount 

 of water lost to the crystallization process (105). Visual observation 

 suggests that hyphal tips are more susceptible than basal mycelial cells 

 and that aerial hyphae are more resistant than submerged (87); the 

 water content may be a factor in these differences. 



Rapid freezing, in which crystal formation occurs throughout both 

 cells and medium, must obviously be so rapid as to limit the size of 



