424 PHYSICAL AGENTS 



all cells are killed in 10 minutes, has been determined for spores of 

 several fungi as 40-60°C, a value close to that usually reported for 

 vegetative cells of mesophilic bacteria (1, 19, 21, 24, 88, 128). How- 

 ever, spores of some species are much more resistant; known examples 

 are the conidia of Nocardia sebivorans (38) and ascospores of Bys- 

 sochlamys fulva (70), Neurospora crassa (40), and Penicillium sp. (153), 

 Conidia of thermophilic actinomycetes appear to be heat-resistant 

 (37). 



Ascospores are more resistant than conidia in Neurospora crassa (40, 

 125) but are equally susceptible in Endoconidiophora fagacearum (24). 

 Data on mycelium are too scanty for generalization; Fusarium lini is 

 killed in 10 minutes at 55°, but Colletotricluim lini survives 3 or 4 

 hours at this temperature (139). A culture of Nocardia sebivorans 

 withstands 10 minutes at 90° (38), and suspended sclerotia of Penicil- 

 lium sp. are completely killed at 82° only in about 16 hours (153). 

 Spores of Erysiphe spp. are somewhat more resistant to heat than my- 

 celium (157), but comparative studies of spores and mycelium in other 

 fungi have not been made. 



Neither the thermal death point nor the related thermal death time 

 has proved adequate in studies of bacteria (124), and there is no reason 

 to hope that they will prove to be any more satisfactory for work with 

 fungi. Development of a better index will, however, have to wait 

 until we have more knowledge of the order of death. Characteristically, 

 the killing of vegetative cells of bacteria is exponential, i.e., a plot of 

 the logarithm of survivors against time is linear (118, 124), although 

 non-exponential orders of death by heat are known (77, 119). Data 

 on the killing of fungi by heat are few in number; Figure 1 shows a 

 non-exponential order of heat killing of ascospores of Byssochlamys 

 fulva; ascospores of Penicillium sp. respond similarly (153). At lower 

 temperatures the curve becomes distinctly sigmoid, i.e., short exposures 

 have proportionately less effect (128). These few statements refer to 

 studies of spores; sclerotia of Penicillium sp. respond to heat as one 

 would expect in a multicellular structure (153). 



Tentatively, we may propose that the non-exponential order of death 

 of fungus spores reflects genetic heterogeneity of the material, but the 

 data are too limited at present to justify a firm conclusion, and in 

 particular the occurrence of bi- or multinucleate cells may influence 

 the reaction. 



Death by heat is most drastically affected by moisture. Cells die 

 much more rapidly at high temperatures if they are wet (22, 37, 74) or 

 if the relative humidity is high (24, 53). Heat resistance in Bys- 

 sochlamys fulva is affected by the pH of the menstruum (70). 



