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pathologists as this. Possibly no disease which has appeared since 
the advent of Phytophthora infestans in the forties presents a greater 
menace to potato culture. 
The literature on leaf-roll has become so voluminous that few will 
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 bulletin 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 Schiumberger (1911), whose critical, 
summary of the literature 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. (PL IV, fig. 1, and PL V and PL 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- 
nounced 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 Schiumberger state of this color character that, according to the 
