POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 19 



pathologists as this. Possibly no disease which has appeared since 

 the advent of Pliyto'pMhora infestans in the forties presents a greater 

 menace to potato culture. 



The literature on Isaf-roll has become so voluminous that few wiU 

 undertake to peruse all the contributions, which are, indeed, of very 

 uneven merit, and anyone who attempts it is likely to emerge with his 

 concepts of the disease more confused and hazy than at the start. 

 • This bulletm is intended as a guide in. the diagnosis of leaf-roll and 

 a summary of present knowledge. It is the result in part of the 

 writer's personal investigations, but much is owed to other writers, 

 and particularly to Appel and Schlumberger (1911), whose critical 

 summary of the Hterature on this disease is commended to all readers. 



DESCRIPTION OF LEAF-ROLL. 



Leaf-roll is a disease characterized by an upward rolling of the 

 leaves, by a decreased yield of tubers, and by transmission of the 

 diseased condition through tubers planted. Its symptoms vary so 

 much in detail that they can be most clearly outlined by separate 

 treatment. 



The rolling of the leaves is the most constant and conspicuous 

 symptom of this disease. The leaflets curl or roll upward on their 

 midrib, often assuming a nearly tubular shape, and giving a plant a 

 staring appearance (PL VII). This rolling is sometimes restricted 

 to the upper leaves, while in other cases all or nearly all of the leaves 

 on the plant exhibit it. (PI. IV, fig. 1, and PI. V and PI. VIII.) 

 This type of roll is distinct from the curly-dwarf condition described 

 on page 37, but a very similar roll may be induced by other causes, 

 such as wet soil, blackleg, and other diseases, as shown on page 26. 



The color of the foliage changes with the advent of leaf-roll, but 

 these color-symptoms vary greatly, from cases where the leaves 

 assume an unhealthy, light-green color to those marked by pro- 

 noimced yellowish, reddish, or purplish colors. These variations 

 appear to depend in part upon the severity of the disease, but they are 

 also to a large extent varietal reactions. The Peachblow, for example, 

 develops considerable red in the upper leaves while the Pearl under 

 the same conditions turns yellowish green. In general, early stages 

 of leaf-roll may not be much yellowed, while more advanced cases, 

 and particularly those in the second or third year, i. e., grown from 

 the tubers of diseased plants, are likely to be quite yellow with red- 

 dish or purplish tints. The development of reds and purples will 

 probably be found to take place in the different varieties according 

 to the natural pigmentation of the sprouts and stems. The greatest 

 variety of colors could be observed in the several numbers of the 

 collection of seedlings which became affected by leaf-roll. Appel 

 and Schlumberger state of this color character that, according to the 



