PARASITISM AND RESISTANCE 



379 



buffered media at pH 4.25 than at 5.25, although growth was more rapid 

 in the less acid medium (Fig. 76). The toxin was shown to be irreversibly- 

 inactivated at pH G or above. The introduction of calcium hydroxide 

 into trees and the application of basic chemicals to the soil have been 

 somewhat successful in retarding the disease. Presumably, these chem- 

 icals act by raising the pH of the sap of the tree. 



Days 

 Fig. 76. Growth of Ceratostomella ulmi and production of toxin, as measured by wilt 

 of tomato seedlings induced by culture filtrate, in buffered media at different pH 

 levels. Note that toxin production is favored by the more acid medium, while 

 growth is greater in the less acid medium. (Courtesy of Feldman, Caroselli, and 

 Howard, Phytopathology. 40: 348, 1950.) 



The varieties of oats susceptible to toxic culture filtrates of Helmin- 

 thosporium victoriae were the same that were susceptible to the fungus in 

 nature (Meehan and Murphy, 1947). Plants of Boone variety were 

 killed, but Clinton plants were unaffected when grown in the same con- 

 centrations of the filtrate. The toxic substance, which was not identified, 

 was produced when the media contained either organic or inorganic 

 nitrogen. This species differs from H. sacchari, which was reported by 

 Lee (1929) to reduce nitrates to nitrites, which were toxic to sugar cane. 



