Oct. II, 191S Phytophthora infestans in Irish Potato 79 



The controls remained free from infection. Because of possible contami- 

 nation, no further observations were made in the remaining plants in 

 this pot. 



This experiment was repeated, beginning February 22, 191 2, but 

 instead of large pots six boxes 18 by 18 by 6 inches were employed. 

 Diseased tubers were planted in four of these and sound tubers in the 

 remaining two. Eight were planted in each box, the conditions being 

 exactly the same as in the preceding experiment. 



On March 3, or 11 days after planting, one shoot was found just break- 

 ing through the soil in one of the two boxes at 23° to 27° C. It seemed 

 perfectly normal both in color and in size, but on examination the next 

 day both the shoot and the surface of the soil immediately surrounding 

 it were covered with a white glistening fungous growth resembling that of 

 P. infestans. Upon examining this growth microscopically it was found 

 to be the potato fungus, as suspected. The mycelium on the soil had 

 grown out from the infected shoot and seemed to be confined to the surface 

 of the soil. The soil about the shoot was removed and the underground 

 portion of the stem exposed. It was found to be water-logged just below 

 the surface of the soil and was gradually becoming brownish as the parent 

 tuber was approached. An examination of the parent tuber showed it to 

 be badly decayed at one end, but quite firm at the other. The tissue of the 

 tuber was examined at the base of the young shoot and showed the char- 

 acteristic blackening due to P. infestans. After 48 hours in a moist 

 chamber the fungus fruited profusely. Plate IV, figure 3, shows a cross 

 section of the tuber and the infected shoot. 



Moisture. — As stated earlier, moisture influences in some way the 

 behavior of the seed tuber and the fungous mycelium contained therein. 

 It was thought worth while to hold infected tubers in comparatively dry 

 rather than very moist soil, as was done in the preceding tests. To this end 

 24 infected tubers with several living eyes each were planted in steam- 

 sterilized soil on January 13, 1914, in a house where the temperature 

 varied from 15° to 20° C. After 30 days they were covered with a glass- 

 house and kept well watered. Ten of the tubers rotted in the ground 

 before producing any shoots. Thirteen days later a small, spindly shoot 

 growing from one of the tubers showed discoloration just at the surface 

 of the soil. This infection spread upward and the fungus fruited the 

 following day. The remaining 13 were allowed to stand two weeks more, 

 but none of them became infected. When dug up, it was found that all 

 the mother tubers were rotten except two. In these P. infestans fruited 

 when the tubers were cut open and laid in a moist chamber, showing 

 plainly that the fungus may remain alive in the parent tubers for at least 

 two months under the conditions of this experiment and also that the 

 mycelium may spread up the stem, even though the infected tuber is not 

 held continuously in wet soil. 



