1 3 . The Action 



of 

 Physical Agents 



i 



_t is convenient, although probably not defensible in 

 the final analysis, to separate the physical and chemical agents that ad- 

 versely affect fungi. Of the several agents of the first type, attention 

 must be focused upon temperature and radiation. Osmotic pressure 

 has been touched upon earlier, in Chapters 1 and 12. Hydrostatic pres- 

 sure has hardly been studied in relation to the fungi; very young in- 

 fections by plant pathogens are prevented from developing by the 

 application of pressure (156), and reduced pressure somewhat fa- 

 cilitates germination in Erysiphe graminis, perhaps by accelerating 

 carbon dioxide loss (12). It may be expected that studies of cellular 

 reactions and of the action of fungicides under high pressures will 

 contribute to our understanding of the fungi, as similar studies have 

 contributed to the knowledge of bacteria (76). 



1. HIGH TEMPERATURE 



The practical problem of heat sterilization is primarily that of killing 

 bacterial endospores; fungal structures, with some exceptions, are less 

 resistant. Consequently, there has been little interest in fundamental 

 studies on the death of fungi at high temperature. However, destruc- 

 tion of fungi within host tissues has proved practicable for a few patho- 

 gens which are present as mycelium in seeds or cuttings (148). 



The thermal death point, defined as the least temperature at which 



423 



