74 
Journal of Agricultural Research 
Vol. V, No. s 
SPREAD OF THE MYCEUUM INTO THE SPROUTS 
When it had been shown that the mycelium was alive in the tuber, at 
least at some point, its spread into the sprouts was studied. Three boxes 
(i 8 by 18 by 6 inches) were filled half full of soil which had never grown a 
crop of potatoes and which had been steamed for 40 minutes in an auto¬ 
clave at 15 pounds' pressure. Twelve tubers were partially buried in 
each box, four of which were sound, the remaining eight being infected 
with P. infestans when harvested. The soil was well moistened with dis¬ 
tilled water, and each box covered with a pane of glass. Each box in the 
series was held at a different temperature—that is, 15 0 to 20°; 20° to 22 0 ; 
and 23 0 to 27 0 C. 
The 8 infected tubers subjected to ^ temperature of 15 0 to 20° produced 
many sprouts, 5 of which became infected during the period under obser¬ 
vation. The tubers subjected to 20° to 22 0 also produced 5 infected 
sprouts, these appearing during the first 14 days after planting. The 
greatest number of infections were obtained from the 8 diseased tubers 
held at 23 0 to 27 0 , 13 sprouts becoming infected during the first 14 days 
after planting. The checks remained free from infection. P. infestans 
seldom sporulated on the parent tuber unless the corky layer was broken, 
but it was very common on the basal portion of the sprouts growing from 
infected tubers. In many cases the eyes producing infected sprouts were 
cut out to learn whether the fungus was present in the tissues imme¬ 
diately surrounding them, and in every case it was found. This showed 
that the sprout infection was due to the spread of the mycelium and not 
to spores present in the air, for had the infection been due to spores the 
checks would have shown as high a percentage of infection as the diseased 
tubers. Infection by P. infestans occurred on sprouts of all sizes, from 
those barely visible to those nearly 1 inch in length. It was a very 
common occurrence to find the fungus sporulating first on the lower 
third of the sprouts, while on the upper two-thirds it was not apparent, 
but it required only one or two days for the remaining portion to become 
covered also, which indicates the rate of spread of the mycelium in the 
sprout tissue. 
Naturally discoloration and decay followed the fructification of the 
fungus. Plate IV, figure 2, shows a potato with diseased and healthy 
sprouts. This is a late stage of sprout infection, and the tissues of the 
two infected shoots have blackened. The healthy sprout stands on a 
portion of the tuber showing no external evidence of the disease, while 
that part surrounding the diseased sprouts is infected with P. infestans . 
The fungus sporulated only on the sprouts of the diseased tubers, while 
those arising from the healthy tubers in the same box remained sound 
throughout, which makes it certain that infection was not by spores 
present in the air or soil, but by the migration of the mycelium in the 
tissues of the parent tuber. 
