S CIRCULAR NO. 109, BUREAU OF PLANT INDUSTRY. 



base of the stem, while numerous rudimentary tubers are formed on 

 the stolons. 



The browning of the woody part of the potato stem and the pres- 

 ence of a brown discolored ring at the stem end of the tubers is not 

 so much a character of the leaf-roll, but is rather to be taken as an 

 indication of the presence of another disease, the Fusarium wilt. 



The formation of aerial tubers is sometimes a feature of leaf -roll, 

 but in other cases is the result of stem cankers caused by the fungus 

 Rhizoctonia. 



Leaf-roll is considered to be hereditary through the seed potatoes; 

 that is, if potatoes borne on plants affected by leaf-roll are planted 

 the resulting crop will be diseased and usually much worse than the 

 first crop. 



The cause of leaf-roll remains unknown, though it has been pre- 

 valent in Europe smce 1905 and has been given much study there. 

 It is now believed to be a physiological disorder rather than one 

 caused by a parasite. Many consider it due to some unfavorable 

 soil or climatic condition, but no one has been able to show what 

 conditions produce it or how it may be controlled by any cultural 



practices. 



LEAF-ROLL AN UNSOLVED PROBLEM. 



No fully satisfactory remedy for leaf-roll has been discovered. It 

 presents one of our most serious problems for investigation and one 

 which it is hoped to push actively as soon as means are provided. 

 We have, however, the benefit of seven years of German experience 

 with the same trouble. The anxiety caused by its appearance in 

 Germany has been somewhat allayed with the passage of time, and 

 the best authority on potato diseases there even states that through 

 the awakening of interest in better culture and in improvement of 

 seed the leaf-roll will prove in the end a benefit to German agricul- 

 turists and their potato production will be permanently increased. 



CONTROL MEASURES. 



While we can not recommend any preventive treatment with the 

 confident assurance that it will be successful, there are some points 

 of attack that are strongly to be recommended as having given the 

 best results elsewhere and as being common-sense measures whether 

 disease is present or not. Most important of these are good seed, 

 crop rotation, and improved culture. 



The seed problem takes first place in any movement for the better- 

 ment of our potato industry and particularly in these western dis- 

 tricts, where diseases are extensively carried on seed. While there 

 may be occasional apparent exceptions, it is the general rule that 

 seed from fields affected by leaf -roll will give a diseased crop. It is 



[Cir. 109] 



