88 BULLETIN 04, U. S. DEPARTMENT OF AGRICULTURE. 
The stem and its branches, the leai 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 curly-dwarf is 
readily distinguished from leaf-roll by the wrinkled or downward 
curling 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, figure 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 mycelium 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 Arlington greenhouses that winter. The 
tubers from diseased hills 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 hills in 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 
hills 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. 
