Oct. II, 191S Phytophthora infestans in Irish Potato 81 



through the winter. The tubers selected showed various stages of 

 infection; but none were used that did not show at least one living eye 

 (bud). On June 6 the tubers were planted in a 2-acre field of potatoes 

 somewhat isolated from adjoining fields, 256 being planted whole in two 

 rows 8 rods long. In a third row 162 hills were planted with cut infected 

 seed. Alternating with these, three rows w^ere planted with healthy 

 seed, Green Mountain variety, as checks. The seed was planted between 

 I and 2 inches deep and the row hilled up so as to cover the sets from 

 3 to 5 inches. Continuous records were taken of the soil temperature 

 by means of a self -registering Richard soil thermograph. A record was 

 also kept of the rainfall, especially as to the date and approximate 

 amount. 



As would naturally be expected, the infected whole tubers sent up 

 shoots more rapidly than the cut seed. Six of the whole tubers had 

 shoots through the ground two weeks after they were planted. On 

 July 6, 30 days after planting, 63 per cent of the whole infected tubers 

 had shoots up; so also did 49 per cent of the cut infected seed and 97 

 per cent of the tubers planted in the three control rows. After July 

 6 the percentage increased very little in any of the foregoing cases. 

 On this same date six of the whole diseased tubers that had failed 

 to send up shoots were dug up for examination. Four of these were 

 dead and nearly decayed, while the remaining two had two and 

 five shoots, respectively, which were just ready to break through the 

 surface of the soil. Plate VI, figure 2, shows the condition of one of 

 these shoots immediately after digging. They were taken to the lab- 

 oratory later and examined for spores of P. infestans, but none were 

 found. Subsequently they were placed in a moist chamber overnight, 

 and the next morning small patches of conidiophores bearing spores, 

 which on microscopic examination proved to be those of P. infestans, 

 were found scattered over the diseased areas. The infected shoots were 

 very much like those obtained in the laboratory experiments discussed 

 earlier. It should be noted that a few days before the plants were dug 

 up a light shower of rain had fallen, which, it is believed, materially aided 

 the progress of the fungus. These developments in the field experiments 

 are wholly comparable with those in the laboratory, in which the sprouts 

 were attacked and overrun by the disease before reaching the surface of 

 the soil. 



On July 13 a very interesting case developed in the row planted with 

 infected cut seed. When the infection was first noted, the discoloration 

 had extended up the stem of the plant only half an inch above the surface 

 of the soil. There was no evidence of spores of P. infestans. The weather 

 was clear and the humidity unusually low, a condition not favorable for 

 sporulation of P. infestans. The plant was carefully watched the fol- 

 lowing day, but no evidence of sporulation could be detected. The next 



