Apr. i, 1921 
Leafroll, Net-Necrosis , and Spindling-Sprout 
55 
planted in a small plot with the nearest groups of leafroll plants culti¬ 
vated with different tools and situated at lower elevations at distances 
of 10, 15, and 40 meters, respectively. The records indicated that no 
leafroll plants had been grown in the soil of this plot since 1916 or possibly 
before, and stocks grown in it during 1917 showed no leafroll in 1918 or 
1919. Infection from the soil of the plot thus seems to be impossible. 
No leafroll was present in the plot during 1919, or in the progeny of 
another bushel lot of the same strain planted elsewhere, 1 except in the 
progeny of 2 of the 70 tubers. These were grown in two insect cages as 
3-hill tuber units. On July 26, spinach aphids (Myzus persicae Sulz.) 2 
were transferred to these 2 tuber units, respectively, from two caged 
potato plants grown in pots, the first having leafroll and the second 
both leafroll and mosaic. The aphids were of a stock originally kept 
upon radish plants for a number of generations and proved to be non- 
virulent in regard to potato diseases (19, p. 262-264). After their intro¬ 
duction to the 2 tuber units, the latter before being dug on September 17 
showed the external characteristics of leafroll in the top leaves. In 
1920 the tubers from these plants were planted with others from the same 
plot, each being split and producing a 2-hill tuber unit. The observations 
made in 1920 are summarized in Table III. 
Table III .—Results of inoculation with aphids from leafroll plants in field cages in IQIQ 
Tuber units in 1919. 
Tuber units in 1920. 
Total. 
Treatment. 
Total. 
Percentage 
of leafroll. 
42 
7 
Caged. No aphids from leafroll plant. 
SOO 
84 
a 8 
O 
Uncaged. Aphids uncontrolled. 
0 
i 
1 
Caged. Aphids from leafroll plant. 
63 
86 
1 
Caged. Aphids from plant with leafroll and mosaic. 
b 7 
° One tuber, both severely net-necrosis and leafroll. For appearance of a leafroll plant of this lot, see 
Plate 9, A. 
6 57 per cent mosaic. 
Nineteen of the 70 tuber units in this plot are not considered in Table 
III, being discarded in 1919 at digging time or their progeny being 
planted in other fields in 192.0 and showing no leafroll. It is clear that 
the introduction of aphids from leafroll plants caused infection and was 
the only means of transmission of this disease in this plot. 3 Plate 9 
shows plants of the second generation of this stock. 
1 Observed by Dr. W. J. Morse. 
* Identified by Dr. Edith M. Patch. 
3 After the manuscript for this paper had been submitted for publication, a similar and contemporary 
but entirely independent experiment was reported as giving similar results (Botjbs, Jan Gerhardus 
Oortwijn. db bladroi,zibktb van db aardappbi.plant . . . viii, 136 p., 8 pi. Wageningen. 1920. 
Inaug. Diss. Literatuur, p. 118-122. Summary in German, p. 123-136). 
