THE POTATO 199 



appear as dark brown bodies, irregular in outline 

 and varying from a mere speck to the size of a grain 

 of wheat. These spots resemble dirt, but do not 

 wash off readily. 



Prof. F. M. Rolfs found three distinct stages in 

 the development of the disease in Colorado, as 

 follows : 



1. Rhizoctonia stage — the first or growing 

 (vegetative) stage. Two kinds of hyphae occur — • 

 the light colored ones in the inner tissue of the host, 

 which, if abundant, produce wet rot; dark colored 

 ones in the outer tissues forming a close web of 

 felted covering, which constitutes merely a girdle or 

 band. If the last only is present, the plant is not 

 killed, but may seem unusually healthy. 



2. The Corticium stage. It had been sup- 

 posed that the fungus produced no spores, but was 

 perpetuated solely by the sclerotia, which are the 

 closely compacted masses of the mycelium forming 

 the dark scale-like or grain-like bodies on the tu- 

 bers and stems of the host plants. At one stage in 

 its development, however, spores are formed on 

 short lateral branches arising from the hyphae of 

 the rhizoctonia stage. These are so readily dis- 

 lodged that their presence is easily overlooked 

 when a microscopic examination is made. It is 

 probable that the spores serve merely for the rapid 

 dissemination of the disease during its vegetative 

 period. 



3. The Sclerotium stage. This is the period 

 when provision is made for the perpetuation of the 

 fungus. The sclerotia on the tubers of an infected 

 crop, on the stems of the potato and weeds, carry 

 the disease over from year to year. 



To stamp out the disease the sclerotia must be 

 killed, and this is done by the use of clean seed, 



