42 BULLETIN 64, U. S. DEPARTMENT OF AGKICULTURE. 



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 

 (»f 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 iij the ^Titer's 

 observations to date, while the former is more frequent m the mosaic 

 diseases of tobacco, tomato, and other plants. Reference to Plate 

 XVI, figure 1 wiU 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 

 \^Tinkled foliage, were it not for the contrast with the healthy plants 

 alongside. (PL XYl, fig. 2.) 



Cases of potato mosaic have been observed mth 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 aU the plants of which had variegated green and yeUow- 

 white leaves. This sort, appropriately named the ''Harlequin," 

 was grown in 1911 in a variety test on the experimental grounds 

 of the Land^virtschaftUches Institut at Goettingen, Germany. As 

 might be expected, it was lacking in ^agor 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 foHage. This effect is manifestly due to the imperfect 

 development of the diseased portions of the leaf parenchj^ma. The 

 plants are also smaller, except in the mildest cases. The effect on 

 the jdeld was tested by harvesting 80 mosaic hiUs 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 -wiU doubtless be 

 cleared up later when both diseases have been more fuUy 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- 

 man}', where it was not uncommon, especially on some varieties. 



