218 Report of the Department of Botany of the 



recommended for the prevention of scab. The effect of the treat- 

 ment was determined by comparing the growth of the Rhizoctonia in 

 artificial cultures made before and after treatment. In this respect, 

 our methods differ from those used by previous investigators. 

 Heretofore, the efficiency of the treatment has been determined, in 

 most cases, by means of field experiments and the results have been 

 more or less unsatisfactory. They have been affected by outside 

 factors such as climatic conditions, texture and drainage of the soil, 

 insects and other fungus diseases. By the culture method such 

 factors are eliminated. 



THE DISEASE. 



The disease under consideration is that caused by Rhizoctonia 

 solani Kiihn which is the sterile form of Corticium vagum B. & C. 

 var. solani Burt. 1 It has been known in Europe for many years. 

 Its occurrence in America was first reported by Duggar and Stewart 2 

 in 1901, and it is now found in all portions of this country where 

 the potato is extensively grown. It is recognized on the surface of 

 the tubers by the masses of resting mycelium or sclerotia. These 

 sclerotia-like bodies are often overlooked, for they may be mistaken 

 for soil clinging to the potato. By immersing the tubers in water 

 or washing them free from all soil, the sclerotia can readily be dis- 

 tinguished as dark brown bodies which cling tenaciously to the 

 surface of the tuber. Plate III shows a tuber with the sclerotial 

 bodies upon it. When the fungus is in this form it is of little 

 importance as the tubers are not injured by it, but affected tubers 

 may disseminate the disease, and in very bad cases the market 

 value of the tubers may be lowered. 



To this fungus is attributed, to a great degree, the so-called 

 " skips " in the planted row. Rolfs, 3 in 1902 and 1904, held it re- 

 sponsible for the failure of potato crops in Colorado. The fungus 

 may live on the organic matter in the soil or it may live as a par- 

 asite attacking the stems, stolons, or roots of the plants. At the 

 point of attack, cankers are formed which in time may increase so 



1 Probably the same as Hypochnus solani Prill. & Delacr. (Clinton, Conn. Sta. 



Rpt. for 1904 : 326; Riehm, Mitt. K. Biol. Anst.f. Land u. Forstw, n : 23. 1911). 



2 Duggar, B. M., and Stewart, F. C. The Sterile Fungus Rhizoctonia. Cornell 



Sta. Bui. 186. 1901; also N. Y. (Geneva) Sta. Bui. 186. 1901. 

 3 Rolfs, F. M. Potato Failures. Colo. Sta. Bui. 70. 1902; and Bui. 91. 1904. 



