Oct. n, 191S Phytophthora injestans in Irish Potato 77 



Of the 78 plants that came up 21 were markedly abnormal, while 

 the remaining 57 were quite sound. The sickly plants were covered with 

 bell jars for several days at a time so as to make the moisture conditions 

 more favorable for P. infestans, but not a single case of infection either 

 on the basal portions of the stems or on the foliage was found, although 

 the plants were examined daily until the vines died down. 



From these experiments and others of a similar nature not mentioned 

 here, it is plain that environmental conditions and the stage of develop- 

 ment of the tuber planted determine whether the mycelium may or may 

 not grow up into the shoots. The conditions prevailing in the ordinary 

 greenhouse are not suited to the spread of the mycelium up into the 

 stems. 



Believing temperature and moisture to be the chief environmental 

 conditions bearing on the development of P. infestans, experiments were 

 made to determine the influence of these factors on the disease. 



Temperature;. — ^The influence of temperature was considered first. 

 Three experiments were made, and, as all were practically the same, a 

 description of one will suffice. 



Five 12-inch pots were nearly filled with soil and steam-sterilized. 

 On January 29, 1912, three tubers infected with P. injestans were planted 

 2 inches deep in each of three of these pots, and in the two remaining 

 pots sound potatoes were planted as controls. Two of the pots were 

 placed in a greenhouse where the temperature varied from 15° to 20° C, 

 depending upon the time of the day; the third was placed in another 

 greenhouse where the temperature ranged from 23° to 27° C. With 

 each was placed a pot containing healthy tubers. 



The first shoot to appear in the pots kept at 15° to 20° C. came up on 

 February 6, or 8 days after the tubers were planted. The healthy tubers 

 used as controls did not come up as soon as the diseased ones. They 

 were more dormant at the time of planting. It has been observed by 

 several investigators that tubers infected with P. infestans germinated 

 sooner than healthy ones. In 12 days all of the diseased tubers had 

 shoots up so high that the panes of glass covering the pots had to be 

 removed. In order to keep the young potato plants in a moist atmos- 

 phere, a large bell jar was placed over each of the three pots. Careful 

 examination was made daily. On March 18, or 45 days after planting, 

 the plants were 7 inches tall, but showed no signs of P. infestans. At 

 this time the plants held at 15° to 20° C. were dug up to learn the condi- 

 tion of the diseased tubers planted. Three were wholly decayed, while 

 the other three were only half rotten and showed on the remaining 

 portion the typical shrunken areas so characteristic of this fungus. 

 All of the tubers in the control pot were sound. The three tubers 

 partially decayed were now placed in a moist chamber in order to ascer- 

 tain whether the fungus was still alive in them after being buried 45 days 



