Oct. ii, 1915 
Phytophthora infestans in Irish Potato 
91 
The conclusions to be drawn from this experiment are perfectly obvious. 
(1) An epidemic can be started by the infection of two plants in a field; 
(2) two infected plants can spread infection sufficiently to destroy the 
vines on a half-acre plot in 29 days. That a larger plot, indeed a field of 
many acres, could be destroyed by one infection is clearly evident. 
It might be argued that these conditions were not typical of those 
occurring under field conditions. On October 14 a visit was made to the 
potato fields of western New York, where an epidemic was just starting 
in many of the fields. Infection centers like the one produced by arti¬ 
ficial infection in the potato plot at Madison were in evidence in several 
fields. Another visit to the 
same fields early in Novem¬ 
ber showed that they had 
been destroyed by an epi¬ 
demic of late-blight. 
The development of late- 
blight under field condi¬ 
tions was again followed 
in the fall of 1913 at Houl- 
ton, Me. Careful watch 
was kept on several fields 
in that vicinity. The first 
infection by P. infestans 
was found in the field on 
August 8, following a few 
days of wet weather. By 
going through nine differ¬ 
ent fields six other centers 
were found. One typical 
case will serve to illustrate 
the prevailing conditions at 
each center. The infected 
leaves were always the 
lower ones of the plant. At 
the center of the infected area the infections were much more numerous 
than elsewhere, probably about ten times as numerous. These centers 
of infection varied' from 8 to 40 feet in diameter. If the centers had 
not become too large, a hill could usually be found that was nearly 
killed and which suggested strongly that it was the point where the 
primary infection originated. • From August 15 to 28 the weather was 
hot and dry, and during this period the fungus made little headway. On 
the date last named a rain fell which facilitated the spread of the disease 
and caused it to become general though not markedly destructive in the 
5771°—15 - 3 
Fig. 3— A cross section of the cortical region of a potato stem, 
showing the mycelium of Phytophthora infestans. This plant 
became infected by the mycelium spreading up the stem from 
the infected parent tuber. This is an early stage of infection, 
and the tissues of the cortex have not been killed. 
