42 BULLETIN 94, U, S. DEPARTMENT OF AGRICULTURE, 
MOSAIC. 
The potato mosaic is an abnormal condition of the foliage charac- 
terized by a spotted or mottled appearance of the leaves, portions of 
which are lighter green in color and with thinner, less perfectly devel- 
oped parenchyma than the normal. In the later stages, brown flecks 
of dead tissues may appear. These light-green areas vary consid- 
erably in size in different cases, from definite patches of 5, 10, or 20 
millimeters, with fairly distinct demarcation between diseased and 
healthy tissues, to an indefinite punctate type where a thin yellow- 
green spot of leaf tissue merges gradually into the apparently normal. 
The latter has been the more common on potatoes in the writer's 
observations to date, while the former is more frequent in the mosaic 
diseases of tobacco, tomato, and other plants. Reference to Plate 
XVI, figure 1 will make these points clearer than pages of text. There 
are phases of mosaic where it might be inferred that the plants under 
observation were of varieties having naturally irregular, curled, or 
wrinkled foliage, were it not for the contrast with the healthy plants 
alongside. (PL XVI, fig. 2.) 
Cases of potato mosaic have been observed with the abnormal 
leaf areas so large and so clearly marked as to suggest variegations, 
such as are familiar among ornamental plants. True variegations 
occur somewhat rarely in potato foliage, but the writer has seen one 
variety all the plants of which had variegated green and yellow- 
white leaves. This sort, appropriately named the "Harlequin," 
was grown in 1911 in a variety test on the experimental grounds 
of the Landwirtschaftliches Institut at Goettingen, Germany. As 
might be expected, it was lacking in vigor as compared with the 
other varieties. 
The effect of mosaic on the growth and development of potato 
plants is quite marked. Most conspicuous is the irregular, distorted, 
or wrinkled foliage. This effect is manifestly due to the imperfect 
development of the diseased portions of the leaf parenchyma. The 
plants are also smaller, except in the mildest cases. The effect on 
the yield was tested by harvesting 80 mosaic hills and 80 healthy 
hills of the Green Mountain variety, on September 10, 1913. The 
yield of the diseased plants was 94.4 pounds; and of the healthy, 
120.8 pounds, a difference of 22 per cent. 
Typical potato mosaic can not be confounded with typical curly- 
dwarf. The former is marked by abnormalities in the leaf paren- 
chyma while the especial characteristic of the latter is the restricted 
development of the vascular elements. There do occur, however, 
some intergrading forms that present puzzles that will doubtless be 
cleared up later when both diseases have been more fully studied. 
No references to potato mosaic have been found in the literature. 
It was first observed by the writer in 1911 in a field at Giessen, Ger- 
many, where it was not uncommon, especially on some varieties. 
