1915] 



APPEL PHYTOPATHOLOGY AND SCIENTIFIC BOTANY . 281 



were situated behind a chain of hills. The depth of the ditches 

 had changed the water level and prepared the right conditions 

 for an attack of Valsa oxystoma. 



In another paper I have shown the importance of the work 

 of E. Munch. 1 This work is a model as to the manner in 

 which investigations of plant diseases upon a scientific basis 

 should be prosecuted. And, therefore, I wish to come back 

 in a more detailed way to the work of Munch. The fungous 

 diseases of our trees belong, in general, to the most im- 

 portant diseases, and we yearly lose millions on their account. 

 But we did not know the factors upon which the appearance 

 of such diseases rested until these were demonstrated by the 

 work of Munch. It was known that many fungi attack woody 

 plants under definite conditions. Sometimes closely related 

 species of one genus of hosts behave differently and some- 

 times only definite tissues are attacked. Lastly these rela- 

 tions vary in different years or seasons in different localities. 

 The difficulty has been that the cause of this variability was 

 sought in the different soil conditions which might have an 

 influence on the constitution of the tissues of the host, in 

 external injuries — such as sunburn or frost, and in the period 

 of development of the fungus. These factors, however, are 

 not of fundamental importance in the question of the produc- 

 tion or suppression of a fungous attack. 



Munch has proved through numerous experiments that the 

 content of air in the tissues is the determining factor. The 

 greater part of the wood-decaying fungi have a large air re- 

 quirement and are able to grow only when a maximum of air 

 is furnished. In the first place the content of air is dependent 

 on the quantity of water, and the occurrence of this large 

 class of plant diseases depends upon the water supply. Simi- 

 larly, the quantity of solid substance may be of influence. 

 Specimens with narrow annual rings are more resistant than 

 those with broad ones, because there is less room for air 

 in the former. The different annual rings of the same wood 



und 



pflanzen. Naturwiss. Zeitschr. f. Forat- u. Landw. 7:54-75, 87-114, 129-160. 



1909. 



