34 BULLETIN 04, L'. S. DEPARTMENT OF AGRICULTURE. 



leaves, but some physiological defect prevented its prompt transloca- 

 tion to the below-ground tubers, it would be laid up in thickened 

 branches and aerial tubers. 



An interesting and important line of study in pathological physi- 

 ology presents itself m the determination of the ways in which leaf- 

 roll potatoes differ from healthy ones. Doubtless a better knowledge 

 of the nature of leaf -roll will lead to a determination of its cause. Up 

 to the present but little more has been done than to diagnose leaf -roll 

 more accurately and separate it from other maladies with which it has 

 been confused. 



CAUSE OF LEAF-ROLL. 



The hypotheses as to the cause of leaf -roll are numerous but exceed- 

 ingly varied. They have indeed only one point in common — that 

 all are as yet unproved. It has been argued by one that leaf-roll 

 results from the use of unripe tubers for seed; by another, that it is 

 due to the employment of matured tubers for seed; while a tliird 

 beheves that seed from prematurely ripened plants is a cause of leaf- 

 roll. The disease is attributed by some to a lack of mineral elements 

 in the soil, while others advance evidence that it is caused or aggra- 

 vated by an oversupply of these same mineral elements. Poor 

 cultural methods, lack of seed selection, and varietal degeneration are 

 other suggested causes. The struggle between those who beHeve 

 leaf-roll due to fungi and those who tliink it nonparasitic is nearly 

 fought out, x^dth the victory apparently in sight for the latter. Many 

 signs now point to the plant breeder as the one who will finally 

 triumph over this malady. 



The present-day opinions on the cause of leaf-roll may be briefly 

 reviewed. (Appel and Schlumberger, 1911.) 



The relation of fungi to leaf-roll has already been briefly summa- 

 rized. Much more on this point mil be found in the writings of 

 Himmelbaur, of Kock and Kornautb, and of Appel and Schlumberger. 

 (See "BibUography," pp. 44-48.) 



On the question of using mature or immature seed, EQltner (1905) 

 is the leading advocate of the stand that the immature seed stock 

 gives an abnormal growth. On later evidence, he limits this to 

 those potatoes wliich are prematurely ripened by drought or other 

 untoward circumstances. Agamst tliis is to be balanced the very 

 extensive use, witli good results, of immature tubers for planting. In 

 Scotland, particularly, this is held to be the best practice. Hiltner 

 (Appel and Sclilumberger, 1911) further holds leaf-roll to be the 

 result of excessive appUcations of fertilizer of unbalanced composition 

 at the AVTong time. He considers that the concentrated salts, espe- 

 cially potash salts, enter the roots and cause a disturbance in nutrition. 

 Through the presence of these salts in the vessels, the water in tliem 

 is prevented from rising. He thinks that these salts also favor the 



