FUNGOUS DISEASES. 61 



processes whether brought about by the lack of carbon dioxide or 

 water, hght, etc. 



The practical results of studies upon the relation of light to fun- 

 gous diseases indicate that plants which are subject to powdery 

 mildews may be shaded only with great caution, and that, on the 

 contrary, those plants which are made more vigorous by partial 

 shade, especially those which are subject to leaf spots of various 

 types may, if other conditions warrant, be placed under partial 

 shade to great advantage. 



Acidity ast> Alkalinity of Soil. 



In recent years, the relations of plants to acid conditions of the 

 soil have received careful attention. In some instances it appears 

 that acid conditions promote root diseases. A Rhizoctonia disease 

 or stem rot of carnation, and a similar disease of sugar beets seem 

 to be of this class. It was at first suspected that the acid conditions 

 of the soil might be directly favorable to the fungus. An investi- 

 gation of this point has shown, however, that the fungus is wholly 

 indifferent to weak acids or alkalies, and the only alternative seems 

 to be that this acidity renders the host plant more susceptible. If 

 this assumption is correct, we may have an explanation of other 

 more or less similar phenomena. Under certain conditions al- 

 kalinity is favorable to the conquest by the fungus. There is in 

 New England, particularly, and extending westward as far as Ohio, 

 a root fungus which is directly or indirectly stimulated to far greater 

 activity by an alkaline condition of the soil. In Connecticut, 

 Clinton has observed a disease of the tobacco of some importance 

 induced by the fungus, Thielavia basicola. The same fungus has 

 been found by Stewart and others on pansies and sweet peas in New 

 York and elsewhere. Recently, the matter of treatment has been 

 taken up by Briggs as a result of studies in the tobacco fields of 

 Connecticut. He finds that the addition of acids and of acid 

 fertilizers to the soil are in this case sufficient to prevent these 

 attacks. From the evidence available through the culture work 

 thus far accomplished with the fungus, it would seem that the 

 fungus is more or less indifferent to slight changes of acidity and 



