BULLETIN 230. 



THE RHIZOCTONIA DISEASE OF POTATOES. 

 W. J. Morse and M. Shapovalov. 



The fungus formerly known as Rhisoctonia solani Kiihn, 

 more recently as Corticmm vagum var. solani Burt and con- 

 sidered by some writers as identical with Hypochnus solani 

 P. & D. is a common inhabitant of New England potato soils. 

 In fact its occurrence is so universal that it is practically impos- 

 sible in most localities in this part of the country, with which 

 the writers are familiar, to examine a barrel of potatoes with- 

 out finding from a few to many tubers carrying at least some 

 sclerotia of the Rhizoctonia stage of the fungus. Fig. 6i shows 

 a potato so affected. Every housewife who prepares potatoes 

 for the table is familiar with the fungus in this form, but the 

 sclerotia are commonly considered to be closely adhering parti- 

 cles of black soil which for some reason are difficult to wash off 

 from the surface of the potato. The fact that these hard, 

 brownish or black, often flattened, closely appressed bodies, 

 which vary greatly in size and shape, are made up of closely 

 interwoven threads of a fungus which serve as a means of carry- 

 ing this fungus over winter and of distributing it from place to 

 place, is not generally understood by those who grow potatoes or 

 prepare them for the table. 



While the appearance of such potatoes is affected and extra 

 labor is required to wash them, the sclerotia are entirely super- 

 ficial and not even the skin of the tuber is injured. Although 

 Rhizoctonia has been classed as an active parasite of the potato 

 in some localities in this and other countries the senior writer 

 has, in the past, shared the opinion of certain other students of 

 the subject with whom he has conferred that it is not an im- 

 portant factor in the production of potato diseases in New 

 England. Work which we have done and observations which 



