July 14, 
Degeneration Diseases of Irish Potatoes 
83 
It is seen from Series 4 and 5 of Table XIX that the method in which 
the stem was split and a part immersed in the juice (PI. 14, A) was inef¬ 
fective; from Series 6 and 7, that the inserting of capillary tubes full of 
juice (PI. 13, B) was slightly effective; and, from Series 1, 2, 3, 8, 9, and 
10, that the leaf-mutilation method was less effective when applied to the 
lower leaves than when applied to the upper leaves or to all the leaves 
(PI. 13, A). With application to the upper leaves or to all (Series 1,3, 
8, 10) the inoculum was most infectious when applied at Stages II or III. 
The complete ineffectiveness of the stem-immersion method and of the 
leaf-mutilation method at Stage I, and the comparative ineffectiveness 
of the capillary-tube method, and of the leaf-mutilation method applied 
to the lowest leaves (Series 2 and 9), all may be caused by the necessity 
of introducing the mosaic virus in greater quantity, under certain green¬ 
house conditions, than was done by these methods. The length of time 
necessary for infection to reach the tubers is indicated, but will be con¬ 
sidered in a later section of this paper. 
On December 20, 1920, in the Washington greenhouse, spinach aphids 
{Myzus persicae Sulz.), in the., adult stage, were transferred from mosaic 
potato, variety Bliss Triumph, to caged potato plants, variety Green 
Mountain, varying in height from 2 to 15 cm. Different numbers of 
aphids were transferred to 12 series of plants each consisting of four 
potato plants from a single quartered tuber, so that one untreated 
control was reserved for three treated plants in each series. A set of 
six series was treated with aphids numbering 1, 5, or 15 individuals to a 
plant. The remaining set of six series was treated with aphids numbering 
2, 10, or 25 individuals to a plant. When the aphids were transferred 
but one stalk to a pot was allowed to develop. All the treated plants 
grew under cages, a separate cage to each pot, during the entire experi¬ 
ment. 
Eighteen days after aphids were transferred, examination disclosed 
that every treated plant was infested in proportion to the number of 
individuals originally placed upon it; some of the plants treated with 
25 aphids showed very decided injury. At this time the aphids were 
killed with nicotine fumigation. On the plants of the first generation 
distinct mosaic was observed 27 days after treatment with aphids. 
With one exception the observations on the second-generation plants 
confirm those which were made on the vines of the first generation. 
Data on the vines in the second generation are presented in Table XX. 
The results in Table XX indicate that in each of the two groups of 
tuber units there was more infection as more aphids had been intro¬ 
duced. It is not known why 5 and 15 aphids per plant in the 1-5-15 
group produced less infection, respectively, than 2 and 10 aphids in the 
2-10-25 group, or why in most of the series the same number of aphids 
did not always transmit the disease. On the whole, this experiment 
suggests that a larger number of aphids is more effective in transmitting 
mild mosaic. These results also suggest that very few aphids sometimes 
are capable of transmitting the disease. 
On December 22, 1921, six Green Mountain plants from 6 to 21 
cm. in height were inoculated with juice from a rugose mosaic seed¬ 
ling potato. The inoculated plants were kept in the Washington green¬ 
house under the same conditions as those inoculated by a single treat¬ 
ment with mild mosaic and without being placed in moist chambers. 
Also the same strain of Green Mountains was used as in the mild mosaic 
inoculations (p. 48). By January 19, 1922, there was, on five of the 
