Oct. ir f 1915 
Phytophthora infestans in Irish Potato 
75 
This experiment was repeated and has been reported in full in an 
earlier paper (21). Except in one particular, the results were, in general, 
alike. In this case a sprout grew out near the surface of the soil from one 
of the infected tubers. This sprout became infected and the mycelium 
of P. infestans grew out from it into the soil for a distance of about 1 cm. 
This is not a usual occurrence and happens only when conditions are very 
favorable for the growth of the fungus. A slight decrease in the moisture 
content of the soil and the fungus is no longer in evidence, nor does it 
return if the original moisture condition is restored. 
This experiment was again repeated on January 29, but only two sets 
of temperatures, 15 0 to 20° and 23 0 to 27 0 C., were used. The other set 
of temperatures was omitted because the supply of tubers was rapidly 
becoming exhausted, and, besides, it had been shown that temperatures 
between 15 0 and 27 0 were the most favorable. The results were, in 
general, like those already recorded and need no further consideration. 
From this series of three experiments, in which infected tubers were par¬ 
tially buried in moist, sterile soil, it is clearly shown that the mycelium of 
P. infestans in infected tubers spreads from the parent tuber into the 
sprouts, where it may sporulate freely. 
Naturally the next step was to learn something as to the behavior of 
the infected tubers when wholly buried in the soil. To this end 12 sound 
tubers of the Irish Cobbler variety were artificially infected with a zoo¬ 
spore suspension held in contact with a sprout about one-fourth of an inch 
long by means of a ring of paraffin, as shown in Plate IV, figure 2. These 
tubers, together with 6 sound ones as controls, were buried 2 inches deep 
in a box of wet sterilized soil and placed in a saturated atmosphere at 23 0 
to 27 0 C. The tubers had gone through the rest period, and in some cases 
the sprouts were 1 inch long. Eleven days after planting, 4 of the tubers 
had thrown up shoots. The remaining 8 were dug up to learn their con¬ 
dition, and it was found that in every case the fungus had spread into 
sprouts other than the one originally infected. Plate IV, figure 2, shows a 
tuber with the paraffin about the infected eye and the cluster of 5 
sprouts at the bud end of the potato. One of the cluster, it should be 
noted, is free from infection. After the tuber was photographed it was 
cut and the discoloration typical of P. infestans was found at the base of 
the sprouts. That it was P. infestans was further shown by the pro¬ 
duction of spores and conidiophores on the discolored tissue. The fungus 
had spread from the initial point of infection over to the point where the 
cluster of infected sprouts originated from the parent tuber. The four 
shoots that came through the ground were allowed to remain until 
April 30, when they were dug up. These were found to be sound, while 
the parent tubers were totally decayed. The controls remained free from 
infection by P. infestans throughout and developed into normal plants. 
5771°—15-2 
