48 
Journal of Agricultural Research 
Vol. XXV, No. 2 
mosaic mottling appeared on the vines from mosaic seed pieces. This 
test confirms earlier, well-controlled experiments, showing that root and 
vine contact alone do not result in mosaic transmission. 
Further data on contact inoculation will be given when considering 
intervarietal transmission. 
juics-transfer inoculations 
Aphids are a natural means of transmission and when of cosmopol¬ 
itan species like potato aphids, Macrosiphutn solanifolii Ashm., or 
spinach aphids, Myzus persicae Sulz., are usually readily available and 
capable of being multiplied to sufficient numbers under control condi¬ 
tions, at least for a limited number of experiments. The difficulty with 
aphids is chiefly one of supplying the proper control conditions for 
inoculations on a large number of plants in the open field. Aphids also 
cause uncertainty regarding the interpretation of the presence or absence 
of certain symptoms, inasmuch as they often produce dwarfing, mottling, 
chlorosis, wrinkling, streaking, and complete necrosis directly even when 
non virulent, especially if abundant, and as, on the other hand, they may 
be influenced by certain conditions so that they do not accept a new 
host readily. Therefore an artificial method of transmission if effective 
may be preferable to the use of aphids, because of greater ease of appli¬ 
cation to a large number of plants, because of the absence of direct 
inoculation injury on the new growth, and because of the greater uni¬ 
formity of treatment. It also introduces no disturbing factor into an 
aphid-free field or greenhouse as would be done by the introduction of 
uncaged aphids or of aphid cages. It sometimes is necessary when 
aphids fail to increase to sufficient numbers. 
While each method has advantages, both should be used, especially 
with such virus diseases as have been transmitted experimentally with 
neither or with only one. As an artificial method of inoculation, tuber 
grafting is handicapped by the uncertainty as to how many new dis¬ 
eases were acquired by the parent vines while apparently healthy, while 
stem grafts in the open field often fail because of hot, dry weather. 
As a result, the leaf-mutilation method (p. 45) has been used exten¬ 
sively by the writers. 
It has been reported (45, p. 253-54; 40, p. 320-26 ) that mosaic can 
be transmitted by leaf-mutilation inoculation, but there is some uncer¬ 
tainty as to what types of mosaic were involved. The type diagnosed 
as mild mosaic has since been used. 
Such inoculations in 1921, in the open field, and during the preceding 
winter, were intervarietal and will be described later. Some were per¬ 
formed and repeated inside of insect cages within the Green Mountain 
variety, upon 6 hills in as many tuber units. Two hills showed current- 
season symptoms and their progeny, three and four tuber units, respec¬ 
tively, were mild mosaic. The other 4 inoculated hills and 18 other 
caged hills in the same tuber units, were all healthy in both generations, 
originating from healthy control hills caged in 1920. Six of the 18 
other hills were inoculated with the capillary-tube method (p. 45) and 
12 were controls. 
On December 19, 1921, in the greenhouse at Washington, D. C., eight 
potato plants, from 3 to 8 cm. in height, were treated with juice from a 
mild mosaic vine by leaf-mutilation inoculation. Four of these plants 
were then kept in a moist chamber for 24 hours while the remainder 
were outside of a moist chamber. By the end of four weeks, mild 
