Oct. ix, 1915 
Phytophthora infestans in Irish Potato 
95 
tubers produced young plantlets when they were allowed to sprout in 
comparatively dry soil. 
De Bary (4) describes a case which is interesting in this connection 
and serves to emphasize the importance of moisture conditions. A 
potato plant was found which had become infected by P. infestans in the 
parent tuber. Portions of the stem just above the surface of the soil 
were infected and discolored, but dry weather prevented the fungus from 
progressing farther in the tissues or sporulating. This was surely a case 
where moisture checked the fructification of the fungus. Two similar 
cases, which are even more striking as showing the close relation of 
moisture and development of the fungus, are described in this paper. In 
these the fungus grew up the stem to the surface of the soil and infected 
the foliage, but the hot, dry weather checked its further spread. 
It is not necessary that the optimum conditions for the growth of the 
fungus should prevail continuously. This is clear from the author's 
experiments where the tubers were started in dry soil and later trans¬ 
ferred to wet soil and the fungus grew up the stem. Too much emphasis 
can not be placed upon the importance of environmental factors and the 
state of germination of the tuber in the production of diseased plants from 
seed infected with P. infestans. A combination of these three conditions 
is not always prevalent in the open nor in the ordinary greenhouse, which 
may well account for the accumulation of negative data. In this con¬ 
nection may be cited one of several experiments where over 300 tubers 
were planted in a greenhouse, where the moisture and temperature could 
not be readily controlled, and not a single infected plant was obtained. 
Clinton (8), Pethybridge (24, 25), and many others have reported similar 
results from extensive field trials. 
In closing this portion of the discussion it should be pointed out that 
not all infected tubers give rise to infected shoots and become centers of 
foliage infection. In fact, only a small proportion function in this way, 
according to the studies of the author; nor has any method been worked 
out whereby an infected tuber can be made to give rise to infected plants 
such as are shown in Plates VI and VII. Whether the progeny of a diseased 
tuber will or will not become infected is determined by the response of the 
fungus and host, coupled with environmental conditions. It is known 
beyond all possibility of doubt, however, that a certain proportion of the 
diseased tubers planted under field conditions may produce progeny 
which becomes infected by the mycelium growing up the stem. Once 
above the surface of the soil, the fungus may sporulate and cause foliage 
infection on the initial and adjoining hills. Infection spreads rapidly 
from such an infection center and is the forerunner of an epidemic. 
Hecke (12) has also noted this early stage in the development of an 
epidemic. It seems logical to assume that these infection centers start 
from planted infected seed potatoes. 
