38 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 



The stem and its branches, the leal petioles, and even the midribs 

 and veins of the leaves all tend to be shortened in many cases to a 

 very marked extent, and particularly in the upper nodes of the 

 plant, so that the foliage is thickly clustered. The diminished 

 growth of the leaf veins, in proportion to the parenchyma, results 

 in a bullate, wrinkled leaf, often strongly curled downward. There 

 seems also to be a tendency to form more secondary branches than 

 is normal, and as these remain short and have curly leaves the com- 

 pactness of the plants is more striking. The stems are also very 

 brittle. 



COLOR OF THE FOLIAGE. 



The color of the foliage in curly-dwarf is typically a normal green, 

 except that in very severe or advanced cases there is a lighter green 

 or yellow color sometimes accompanied by brown or reddish flecks 

 in the leaves where the tissues are dying. Typical cm^ly-dwarf is 

 readily distinguished from leaf-roll by the wrinkled or downward 

 curUng of the leaves, the normal color of the foliage, and the firmness 

 of the leaves, which do not lack turgidity. 



The tuber yield of curly-dwarf plants is greatly curtailed. Severe 

 cases have no tubers, and many such have been observed. In others 

 a few small potatoes are formed. This difference in productivity is 

 strikingly shown in the photograph reproduced in Plate XIII, figm-e 2, 

 of the yield from curly-dwarf hills compared with adjoining healthy 

 hills. 



The nature and cause of this disease remain unknown. No evidence 

 of fungi or other parasites have been found. There is neither brown- 

 ing nor myceUum in stems and tubers, but the curly-dwarf is trans- 

 mitted through the seed. The hereditary nature of the trouble is 

 attested by the German authorities, and it has been observed by the 

 writer in the case of some hill selections made by Prof. Stuart in 

 1911 and planted in the ArUngton greenhouses that winter. The 

 tubers from diseased hiUs all developed into curly-dwarf plants, while 

 those from healthy hills remained normal. Equally good evidence of 

 the transmission of this diseased condition through the tubers was 

 afforded by the .Stuart seedling collection of 1913, which, as described 

 under leaf-roll, was planted in 2-hill tuber units. No. 4033 had 4 pairs 

 of curly-dwarf and 5 pairs of healthy hiUs m the following order: Two 

 normal hills, 2 curly-dwarf, 2 normal, 2 curly-dwarf, 2 normal, 2 curly- 

 dwarf, 2 normal, 4 curly-dwarf, 2 normal. No. 13016 had the first two 

 hUls normal, the next two curly-dwarf. No. 13372 had 4 normal hills, 

 then 4 curly-dwarf, 1 normal, 1 curly-dwarf, and 2 normal. No. 14637 

 had hills Nos. 1 and 2 normal, and 3 and 4 curly-dwarf; and these 

 examples might be multiplied many times. The few exceptions where 

 single hills developed the disease may be due to an error in dropping 

 the seed or to planting a small tuber whole. 



