376 PHYSIOLOGY OF THE FUNGI 



and toxins but may be weak in mechanical action. Some of these cause 

 rapid rots of fruits or vegetables but are unable to penetrate the unbroken 

 epidermis and must depend on wounds for their entrance. Others, which 

 are seldom, if ever, found as pathogens in nature, may cause rot when 

 artificially inoculated into succulent plant tissues. 



Rotting of the tissue is due to two distinct effects of the fungus on the 

 host: (1) death of the cells, and (2) dissolution of the middle lamellae. 

 The separation of the cells is due to the action of the enzymes proto- 

 pectinase, pectinase, and pectase on the middle lamella. These three 

 enzymes are often collectively referred to as pectinase. There is some 

 evidence that pectinase may also cause a change in permeability of the 

 cell membranes and the death of the cells, but it is possible that some 

 other toxic substance may be closely associated with pectinase. How- 

 ever, no such substance has been isolated. Extracts of rotted tissues 

 have been shown to cause the same effects as the fungi themselves. These 

 effects are described by De Bary (1886) for Sclerotinia sclerotiorum and 

 by Brown (1915) for Botrytis cinerea. Higgins (1927) believes that 

 oxalic acid produced by Sclerotium rolfsii is the principal agent of destruc- 

 tion. The death of the host cells well in advance of the invading hyphae 

 indicates rapid diffusion of the toxic substance in the case of fungi produc- 

 ing soft rot. Brown (1948) believes that the enzyme pectinase acts as a 

 cytolytic toxin. For a discussion of the identity of enzymes and toxins 

 of species of Clostridium, see Smith (1949). 



Thatcher (1942) has shown that B. cinerea and S. sclerotiorum cause 

 a fourfold increase in the permeability to water of the host cells just 

 beyond the discolored necrotic zone. Some substance other than pecti- 

 nase may bring about this change in permeability and be a contributing 

 factor to the "action in advance" of many fungi. PhytophtJwra infestans 

 caused a change in permeability in host cells beyond the extent of the 

 hyphae which penetrated the living tissue. The identity of the substance 

 causing a change in permeability is unknown, but it is likely a weak toxin 

 or an enzyme which alters the structure or activity of the plasma mem- 

 brane. The increase in permeability may concern water alone or both 

 nutrients and water. 



An osmotic pressure higher in the fungus cells than in the surrounding 

 host cells is apparently characteristic of the host-parasite relationship 

 (Table 63) . This is necessary before the parasite can absorb water from 

 the host cells. 



The production of pectinase and its activity under different conditions 

 were studied by Vasudeva (1930) and Chona (1932), who showed that 

 the amount produced by Botrytis allii varied with the medium in which 

 the fungus was grown. B. allii did not secrete a demonstrable amount of 

 pectinase when grown on apple extract, but when asparagine, potassium 



