PARASITISM AND RESISTANCE 



389 



On the other hand, greater acidity of the cell sap may favor the develop- 

 ment of some diseases. The susceptibility of certain varieties of grape to 

 Guignardia bidwellii has been correlated with a greater amount of tartaric 

 acid (Butler, 1918). This author points out that leaves are susceptible 

 only while they are young and rich in tartaric acid. 



Table 64. Hydrogen-ion Readings of Apparently Healthy Cobs of 75 Inbred 



Lines of Corn and Incidence of Basisporium Ear Infection Following 



Both Natural and Artificial Inoculation 



(Redd3^ loim Agr. Expt. Sta. Research Bull. 167, 1933.) 



According to Smith et al. (1946), there is evidence that slight variation 

 in pH may have a greater influence upon disease resistance of a plant than 

 is generally believed. Such resistance is not due directly to the number 

 of hydrogen ions. These authors state: 



The observed behavior of hydroquinone and catechol, representatives of the 

 widely occurring ortho- and para-dihydroxyphenolic compounds, suggested that 

 hydrogen ion differences also may influence toxicity by affecting the rate or 

 extent of conversion to the more toxic quinones on invasion by pathogens or by 

 other injur5^ 



The possibility that the presence of the pathogen may alter the pH 

 of the host cells, making it more favorable to extensive invasion, should 

 not be overlooked. Apparently this situation exists in the relation of 

 Erwinia carnegieana to its host, the giant cactus of Arizona. Boyle 

 (1949) reported that the freshly expressed sap from uninfected plants 

 gave pH readings of 5.0 to 5.5, while the healthy-appearing tissue from 

 infected plants had pH values of 7.0 to 7.4, and the pH of discolored tissue 

 not yet broken down was 8.7 to 9.0. These differences could not be 

 attributed to genetic variation and were believed to be a result of the 

 pathogen. The possibility that similar relations exist between fungus 

 pathogens and their hosts seems to merit greater consideration than it 

 has received. 



