PARASITISM AND RESISTANCE 



377 



nitrate, or ammonium sulfate was added to the apple extract, there was a 

 decided increase in the amount produced. This is not surprising, for 

 there are numerous reports that the available nutrients and the pH of the 

 culture medium affect both the kind and amount of metabolic products of 

 a fungus. 



The activity of the enzymes produced by a pathogen varies with the 

 conditions of the host cells. It seems probable that the inhibition of the 

 fungus enzymes by the host cells is an important factor in resistance. 

 Klotz (1927) proposed this hypothesis to explain the greater resistance 

 of sour orange and the greater susceptibility of lemon to Pythiacystis 

 citrophthora and Phomopsis calif or nica, the causes of certain bark diseases. 



Table 63. The Osmotic Pressures of Host and Parasite 

 (Thatcher, Can. Jour. Research 20, 1942.) 



The greater pathogenic action of a destructive fungus occurs in the host 

 whose cells are favorable for the activity of the enzymes of the fungus. 

 Further evidence of enzyme inhibition of certain plant tissues was 

 presented by Chona (1932), who studied the rotting action of B. cinerea, 

 the cause of a soft rot of various plant tissues, and Pythium sp., a rot 

 producer of potato tubers. Vigorous germination of spores of Botrytis 

 and even some sporulation took place in artificial wounds in potato 

 tubers, but no decomposition followed. The pectinase produced by B. 

 cinerea was active against apple tissue, but the presence of potato tissue 

 inhibited its activity. It was then found that the mineral salts, particu- 

 larly KH2PO4 and MgS04, in the potato were the inhibiting factors. On 

 the other hand, Pythium spores germinated well on apple tissue but failed 



