Oct. ii f 1915 Phytophthora infestans in Irish Potato 79 
The controls remained free from infection. Because of possible contami¬ 
nation, no further observations were made in the remaining plants in 
this pot. 
This experiment was repeated, beginning February 22, 1912, but 
instead of large pots six boxes 18 by 18 by 6 inches were employed. 
Diseased tubers were planted in four of these and sound tubers in the 
remaining two. Eight were planted in each box, the conditions being 
exactly the same as in the preceding experiment. 
On March 3, or 11 days after planting, one shoot was found just break¬ 
ing through the soil in one of the two boxes at 23 0 to 27 0 C. It seemed 
perfectly normal both in color and in size, but on examination the next 
day both the shoot and the surface of the soil immediately surrounding 
it were covered with a white glistening fungous growth resembling that of 
P. infestans . Upon examining this growth microscopically it was found 
to be the potato fungus, as suspected. The mycelium on the soil had 
grown out from the infected shoot and seemed to be confined to the surface 
of the soil. The soil about the shoot was removed and the underground 
portion of the stem exposed. It was found to be water-logged just below 
the surface of the soil and was gradually becoming brownish as the parent 
tuber was approached. An examination of the parent tuber showed it to 
be badly decayed at one end, but quite firm at the other. The tissue of the 
tuber was examined at the base of the young shoot and showed the char¬ 
acteristic blackening due to P. infestans. After 48 hours in a moist 
chamber the fungus fruited profusely. Plate IV, figure 3, shows a cross 
section of the tuber and the infected shoot. 
Moisture. —As stated earlier, moisture influences in some way the 
behavior of the seed tuber and the fungous mycelium contained therein. 
It was thought worth while to hold infected tubers in comparatively dry 
rather than very moist soil, as was done in the preceding tests. To this end 
24 infected tubers with several living eyes each were planted in steam- 
sterilized soil on January 13, 1914, in a house where the temperature 
varied from 15 0 to 20° C. After 30 days they were covered with a glass¬ 
house and kept well watered. Ten of the tubers rotted in the ground 
before producing any shoots. Thirteen days later a small, spindly shoot 
growing from one of the tubers showed discoloration just at the surface 
of the soil. This infection spread upward and the fungus fruited the 
following day. The remaining 13 were allowed to stand two weeks more, 
but none of them became infected. When dug up, it was found that all 
the mother tubers were rotten except two. In these P. infestans fruited 
when the tubers were cut open and laid in a moist chamber, showing 
plainly that the fungus may remain alive in the parent tubers for at least 
two months under the conditions of this experiment and also that the 
mycelium may spread up the stem, even though the infected tuber is not 
held continuously in wet soil. 
