Oct. it, 1915 
Phytophthora infestans in Irish Potato 
99 
It is to be hoped that the recent researches on this problem will afford 
an angle of approach that will yield positive evidence in the near future. 
In closing it should be pointed out that, although P. infestans rarely 
produces oospores in the potato plant, this should not be looked upon as 
abnormal. As shown in this paper, the production of resting organs is 
not necessary for the hibernation of the fungus. The mycelium is quite 
sufficient. There are many species closely related to P . infestans that 
produce few resting spores on certain of their hosts. These may per¬ 
petuate themselves from one season to another by means of the living 
mycelium in the perennial parts of the host plant in much the same way 
as already described for P. infestans. The sparing production of oospores 
and the hibernation of the mycelium are therefore not uncommon in 
several species of this family. 
SUMMARY 
It is clear from the author’s experiments that the mycelium of Phy¬ 
tophthora infestans spreads in the tissues of the potato tuber and finally 
reaches the sprouts. The growth of the fungus is retarded when diseased 
tubers are held in dry soil or at temperatures below 5 0 C. Infected 
tubers rot rapidly when placed in warm wet soil. This explains the 
wide variation in stand obtained by earlier writers. A temperature of 
23 0 to 27 0 C. and a well-watered soil were found to be the most favorable 
for the mycelium to spread in the tuber and grow out into the sprouts, 
both when partially and when wholly covered with soil. Under these 
conditions the sprouts may become infected from 4 to 20 days after 
planting, regardless of their size and age. The time required is doubt¬ 
less influenced by the proximity of the mycelium to the buds and the 
external conditions. 
The mycelium of P. infestans may remain alive in seed tubers planted 
in the soil for at least 45 days, and it is very possible that under con¬ 
ditions less favorable for the soft rots which follow P. infestans in the 
tuber the fungus may live much longer. None of the author’s results 
or observations tend to show that the potato fungus is latent in the stems 
and leaves of plants growing from diseased tubers, as stated by Massee 
(20). 
Laboratory tests showed that the fungus infects not only the sprouts 
but also the shoots that break through the soil. The mycelium grows 
from the tuber into the stem, where it travels up to the surface of the 
soil and sporulates, as held by De Bary (4) and Jensen (14). This 
usually takes place in the small dwarfed shv Hs in a hill. 
Potato tubers infected with P. infestans used for seed purposes and 
planted under field conditions may cause the development of an epi¬ 
demic. The mycelium grows from the parent tuber up into the stem 
exactly as shown in the laboratory experiments. It later sporulates 
and foliage infection results. Ten such cases were found and followed 
