THE RHIZOCTONIA DISEASE OF POTATOES. I95 



Rhizoctonia. Their discussion leads the reader to infer that at that 

 time they considered this occurrence of the fungus on the potato in this 

 country as of minor economic importance. However, they mention one 

 instance observed by IMr. F. M. Rolfs where there was good evidence 

 that the fungus killed a considerable number of potato plants. 



Rolfs (I'l), working in Colorado in 1902, was the first to demonstrate 

 conclusively that Rhizoctonia may be responsible for a serious disease 

 of the potato in the United States. The importance of this and a later 

 publication (12), as bearing upon potato diseases in other parts of the 

 country, apparently has not been fully appreciated by American phyto- 

 pathologists. This is doubtless due to the fact that they felt that in 

 the irrigated districts of Colorado Rolfs was working under conditions 

 somewhat dissimilar to those which obtain in other potato growing 

 sections and, therefore, would be liable to encounter difficulties which 

 would not be expected elsewhere. 



The first publication was simply a preliminary report in which he 

 points out some of the important characteristics of the disease and pre- 

 sents certain evidence tending to show that Rhizoctonia is the cause of 

 conditions observed. It does, however, contain many observations 

 which he was able to confirm later, and which the present writers have 

 been able to duplicate in Maine under decidedly different conditions 

 with regard to soil and moisture. Cases are reported where the plants 

 were apparently thrifty and gave promise of an abundant yield, but at 

 harvest time only a few potatoes were found in a hill. Less frequently 

 the plants set an abnormally large number of tubers and these were 

 small and clustered near the surface of the ground, giving rise to the 

 term "Little potato disease." Another characteristic was the formation 

 of aerial tubers or the production of green, tuber-like out-growths in the 

 axils of the leaves and branches above ground. The author claimed 

 , that these conditions resulted from the attacks of the fungus upon the 

 parts of the plants below ground. He also stated that the young plants 

 were often severely injured or even killed before they reached the sur- 

 face. 



In the second report alreadj^ mentioned Rolfs gives the results of 

 additional studies upon the life history of the fungus, the nature of the 

 injuries upon the host, factors concerned in the spread and propagation 

 of the disease, methods of control, etc. Perhaps the most important 

 contribution made in this paper was the publication of the discovery of 

 the Corticium or fruiting .stage of the fungus. 



Laboratory studies showed that to grow best the fungus required 

 plenty of moisture and a temperature equivalent to that which is experi- 

 enced out of doors in mid-summer. Likewise, according to Rolfs 

 experience, the disease was more virulent where the ground was wet 

 and soggy. This taken in connection with other field observations led 

 him to suggest that over-watering the plants in the irrigated districts 

 materially increased the severity of the attacks of the fungus. 



As regards preventive measures Rolfs found that treating the seed 

 with corrosive sublimate gave marked gains where the crop was raised 



