Oct. ii, 1915 
Phytophthora infestans in Irish Potato 
83 
check rows, and as there was no other evidence of infection in this entire 
row it seemed quite certain that the spores had come from the hill pre¬ 
viously mentioned. On August 5, six days after this stray infection 
was first noted, 14 others were found immediately below it on the leaflets 
in the same hill. It seemed quite apparent that the spores had fallen 
from the infection above and infected the leaves below. The disease 
continued to spread rapidly until August 10, when a period of hot, dry 
weather for 10 days checked its development temporarily. At the end 
of this dry spell, however, it resumed activity, and an epidemic of blight 
was well under way in this portion of the field. All the plants in the plot, 
except those on a few short rows of a foreign resistant variety, were 
killed by late-blight before frost. Four other cases, similar to the one 
just described, developed between July 25 and August 4. The symptoms 
in all cases were the same and need not be repeated. In each case the 
spores produced by the initially diseased shoots infected adjoining 
foliage and became centers for the spread of the disease. 
The plants in the three alternating rows planted with healthy seed 
were watched for evidence of stem and foliage infection as carefully as 
those planted with infected seed, as was also the rest of the 2-acre field, 
but in no case did any infections develop that could not be traced to the 
centers in the rows planted with infected seed. Of course, after the epi¬ 
demic was well under way, the source of any single infection was un¬ 
known. The significant point and the one on which information was 
desired was the origin of the very early stages in the development of an 
epidemic and not the late. 
The results of the field tests of 1913 may be briefly summarized as 
follows: (1) Only 63 per cent of the whole infected tubers and 49 per 
cent of the cut infected seed grew; (2) the mycelium in infected seed 
tubers responded the same way in the field as it did in the laboratory 
experiments; (3) shoots were found that became infected before they 
reached the surface of the soil; (4) others infected were able to break 
through the soil and become centers of foliage infection. On these 
dwarfed infected shoots the fungus fruited and infected the foliage, first 
in the same hill and later in those adjoining. In this way these hills 
became the centers for the development of an epidemic. 
FIELD STUDIES IN 1914 
It is well known that too much reliance can not be placed on the 
-results of i-year trials under field conditions. This is especially true 
when dealing with a fungus like P. infestans , which is very much influ¬ 
enced by environmental conditions. In view of this fact, it seemed 
desirable to repeat the field trials of 1913. In 1914, a plot of ground 
was chosen at Caribou, about 60 miles north of Houlton, Me., where 
conditions are fully as favorable for the development of late-blight as at 
