Phytophthora injestans in Irish Potato 



The conclusions to be drawn from this experiment are perfectly obvious, 

 (i) 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 191 3 at Houl- 

 ton. Me. Careful watch 

 was kept on several fields 

 in that vicinity. The first 

 infection by P. injestans 

 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 infesians. 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. 



