1915] 



APPEL PHYTOPATHOLOGY AND SCIENTIFIC BOTANY 283 



to species of Valsa and other fungi. When, however, such 

 diseases occur, you will find the cause in defective irrigation 

 methods, which may be remedied by changing the irrigation 

 system. It is of the greatest importance that the land be 

 irrigated at the time the trees contain less water and plenty 

 of air, and that the next irrigations be made in time to prevent 

 an excessive decrease of the water in the tissues. 



Not all fungi, however, are dependent upon the air contained 

 in the wood. This is, for instance, the case with Armillaria 

 mellea, where the rhizomorphs bring a sufficient quantity of 

 air into the inner tissues. Whoever has cultivated the fungus 

 artificially knows that after a short time rhizomorphs are 

 formed which grow deep into the medium. But the rhizo- 

 morphs are not formed on all kinds of trees and it may be 

 possible that the fungus in these cases depends on the air 

 already in the wood. 



Another question of great importance for American con- 

 ditions is the question whether the growth of bacteria, prin- 

 cipally of Bacillus amylovorus, is dependent upon the air con- 

 tent of the host or not. These experiments must be sup- 

 ported by thorough physiological investigations. That manner 

 of control which seeks to remove the bacteria by cutting out 

 the branches does not guarantee success for the future. I 

 have been convinced of this in my trip through the United 

 States, where I visited districts in which this control measure 

 was thoroughly carried out. 



It may be possible that not only trees, but also herbaceous 

 plants, show relations between fungous growth and air con- 

 tent. I think it must be so for the organisms which cause the 

 wilt diseases and the rhizoctonia disease of the potato, both 

 of which have a high air requirement. On media poor in air 

 these fungi grow only on the surface and absorb very eagerly 

 the oxygen of hydrogen peroxide. The growth of Rhizoctonia 

 in the well-aerated peat soil of the Stockton Delta and the 

 forest soil of Germany is more marked in the dry years than 

 in the years when the plants get a sufficient supply of water. 

 In the United States these diseases are wide-spread, princi- 

 pally through the irrigated lands. In my trip I came to the 



