388 PHYSIOLOGY OF THE FUNGI 



fungus, while conditions which favor rapid maturity of the host cells may 

 cause the fungus to be left behind in the mature tissues which it cannot 



penetrate. 



Hart (1929) studied the nature of resistance of wheat varieties to stem 

 rust and described a type of resistance that she terms funciio7ial resistance, 

 which is dependent upon the stomatal movements of the host, and con- 

 cluded that the parasite enters the wheat only through open stomata. 

 There has been frequent discussion regarding the importance of the 

 acidity of the cell sap of the host and its effect upon resistance. The 

 effects of cell-sap acidity may be threefold: (1) an increase in the hydrogen 

 ions; (2) the toxicity of the organic acids; (3) the influence upon the 

 chemical changes and the possible formation of toxic products by the host 

 cells. In some cases these effects have not been satisfactorily distin- 

 guished. Numerous examples may be found in the literature in which 

 resistance has been attributed to the acidity of the host or host part. 

 Butler (1918) refers to investigations showing that the leaves of varieties 

 of grape resistant to powdery mildew contain three to five times as much 

 acid as the nonresistant varieties. He also showed that the red rot 

 fungus of sugar cane, Colletotrichum falcatum, was present in infected 

 canes from sowing time but usually did not develop severely until matu- 

 rity of the canes. He attributed this to either the increase in sugar or the 

 decrease in acid. The more acid lemons are less attacked by the fruit- 

 rotting fungi. The amount of acid in the fruit, as indicated by chemical 

 analysis, may be greater than the amount necessary to check the growth 

 of the fungus in culture (Cook and Taubenhaus, 1911). A number of 

 workers have considered cell-sap acidity as a possible cause for resistance 

 of wheat to stem rust, but this factor now is believed to be of little impor- 

 tance. No correlation was found between resistance and acidity of the 

 expressed sap (Hurd, 1924). Similarly, there was no correlation between 

 resistance and hydrogen-ion values or the titratable-acid values of the 

 juice of wheat plants resistant to Ustilago tritici (Tapke, 1929). 



Some of the most complete experimental evidence showing the correla- 

 tion between acidity and resistance is presented by Reddy (1933) for 

 different inbred lines of corn in relation to Basisporium gallarum. Briefly, 

 he found that when the pH of the cob was below 5.0, resistance to cob 

 infection was high. Resistance was notably lower at high pH values. 

 Table 64 gives a summary of some of Reddy's experiments. Reddy also 

 believes that the influence of pH may explain why the seedlings, which 

 are acid, are resistant to infection by B. gallarum, while the dry kernels, 

 which are neutral or alkahne, are susceptible. On the basis of evidence 

 previously discussed, it is likely that the pathogenic activities of certain 

 enzymes produced by B. gallarum are inhibited in media having pH of 

 5.0 or less. 



