Oct. ix, 1915 
Phytophthora infestans in Irish Potato 
89 
healthy and diseased tissue, the hyphae could be readily seen ramifying 
between the cells, as shown in figure 3. The mycelium can usually be 
found higher up in the stem in the cortex than in the pith cells when the 
disease is growing up the stem from the infected parent tuber. When the 
cortex has been destroyed it may be found in the pith cells. So far the 
author has seldom found the mycelium in the vascular system or the 
wood cells. Histological studies indicate that the mycelium of P. 
infestans spreads up the stem most rapidly in the cortical region when 
conditions are favorable 
for its rapid growth. 
DEVELOPMENT OF EPI¬ 
DEMICS OF PHYTOPH¬ 
THORA INFESTANS 
One argument used 
persistently against the 
theory of resting myce¬ 
lium being the means of 
perpetuation of P. in¬ 
festans is the sudden 
and almost simultane¬ 
ous outbreak of the dis¬ 
ease over wide areas. 
It has seemed more 
plausible to many to 
imagine that some form 
of resting spore func¬ 
tioned in spreading the 
disease rapidly each 
year, as is known to be 
the case in related spe¬ 
cies. Massee (20) has 
questioned the capac¬ 
ity of the conidia of P. 
infestans to start an epidemic. He believes that epidemics start from 
mycelium of the fungus latent in the tissues which becomes active with 
the advent of favorable weather conditions. 
In the fall of 1911 the following experiment was made at Madison, Wis., 
to learn something as to the development of an epidemic of P. infestans 
under field conditions, with special reference to the r 61 e played by conidia. 
It should be mentioned that this fungus seldom occurs in the vicinity of 
Madison, and, so far as known, it was absent from the State in 1911. The 
writer is sure it did not occur in the vicinity of Madison that year, and 
therefore his results were not complicated by its presence. On the even- 
Fig. i.—C ross section of a potato plant, showing the mycelium of 
Phytophthora infestans , which has killed the cells of the cortex and is 
a later stage than that shown in figure 3. The mycelium is present 
among the pith cells. The plant from which this cross section was 
made became infected like the one in figures. 
