PECAN ROSETTE. 11 
In varieties where the size and distribution .of the yellow spots 
made it possible, Baur found that by carefully cutting out all yellow 
spots and continuing this process on all newly developing leaves for 
one or two weeks, finally green leaves only were formed. From 
this result he considered it certain that the contagium is present in 
the yellow spots but only in sufficient quantity to infect about three 
or four newly developing leaves at the growing point. After this it 
is apparently used up, and leaves subsequently formed remain green. 
Darkening the assimilating leaves of a mottled plant led to a similar 
result. Here the first leaves to develop thereafter were yellow 
spotted, but if these also were darkened before they began to assimi- 
late, the subsequently developed leaves were all green. 
Girdling experiments demonstrated that the contagium is carried 
only through the bark. Different species and varieties of Malvaceae 
were found to vary widely in susceptibility and also in the incubation 
period, but several days at least are required. Baur also demon- 
strated infectious chloroses in Fraxinus, Laburnum, Sorbus, Ptelea, 
Euonymus, and Ligustrum. 
In potato mosaic the mottling is irregularly distributed irrespec- 
tive of the venation ; and, moreover, profound dwarfing, with curling, 
crinkling, and further distortions of the foliage occur in the more 
severe attacks of the disease. The parenchyma tissues are less per- 
fectly developed in the light-colored areas, the palisade cells tend to 
shorten up and chlorophyll development is restricted. Potato mosaic 
(65, 67) is transmitted by the tubers, by grafting, and by inoculation 
with the expressed juices of diseased plants; and it is also dissemi- 
nated by aphids. 
In potato leaf-roll the leaves become inrolled from the margin, 
reduced in size, and of a paler green to yellowish cast. A necrosis 
of the phloem (60-62) seems to be characteristic of the disease, but 
whether this condition is specific for leaf-roll is still a moot point. 
The disease is transmitted by means of the tubers, by grafting, and by 
insects (66). 
In both potato mosaic and leaf-roll portions of the healthy plants 
growing near diseased plants contract the disease, but not all tubers 
from such secondarily infected plants necessarily develop diseased 
progeny. Affected leaves in both these diseases exhibit deep-seated 
assimilatory derangements, including increase in activity of the oxi- 
dizing enzyms and gorging of the leaves with starch, with greatly 
delayed starch translocation. In both these diseases tuber forma- 
tion is greatly restricted. In neither case are these diseases induced 
or cured by soil or climatic conditions. However, certain environ- 
mental factors may temporarily mask the external signs though such 
diseased plants still retain their power to infect. 
