86 Journal of Agricultural Research voi. v, no. 2 



up for examination, but no spores of P. infestans were found. They were 

 again planted and the next day a rain fell, wetting the ground down to 

 the seed potatoes. On June 30, four days after the second planting, the 

 seed pieces were dug up again. Microscopic examination showed that 

 spores and conidiophores of P. infestans were present on 26 of the 31 

 pieces and the growth of the fungus in seven cases was readily visible to 

 the unaided eye. The spores were found to germinate freely in water. 

 These seed pieces were again planted on July i and left in the ground for 

 a period of 14 days. At this time careful examination revealed a limited 

 number of spores on 5 of the pieces, but these spores did not appear to 

 be normal; and when placed in water only 3 or 4 germinated. A search 

 was also made for mycelium of P. infestans in the soil adhering to 

 the seed pieces, but none was found. The plants that grew from these 

 infected seed pieces were examined daily from the time they came up 

 until the vines were nearly mature, but no infection by P. infestans 

 appeared on the foliage. 



The above experiment was repeated, beginning on July 2. In this 

 test 14 diseased seed pieces were planted just after a light rain. Four 

 days later they were dug up and examined ; on 7 of the tubers spores of 

 P. infestans were found. There was no indication that the mycelium 

 was growing saprophytically in the soil adhering to the cut surfaces of 

 the diseased pieces. The pieces were immediately replanted and allowed 

 to grow throughout the season. On July 25 the stem of one of the plants 

 showed infection at the surface of the soil. When dug up, it was found 

 that all of the stem below the surface was diseased and also the parent 

 tuber at the point where the stem originated. This tends to show that 

 the mycelium grew from the parent tuber up into the young shoot and 

 that the infection was not caused by spores in the soil. This plantlet 

 stood in an exposed place and soon died. Spores were produced, how- 

 ever, and a leaf on an adjoining plant became infected. This spread 

 slowly in the leaflet and only a few spores were produced. Finally the 

 leaflet died and dried up and no further infections occurred on any of the 

 plants in the same or adjoining rows. In both these experiments conidia 

 were produced on the seed tuber, but none of them functioned in causing 

 any infections. 



In the spring of 1914 further tests were made at Caribou, Me. On 

 June 4, 183 potato seed pieces infected with P. infestans were planted in 

 accordance with common practice. The next day it rained. On June 7, 

 26 of the 183 seed pieces were dug up and examined for conidiophores and 

 spores of the fungus. These were found on 9 of the pieces and the growth 

 was abundant enough to be easily seen with a hand lens. On July 10, 

 12 more seed pieces were dug up and examined, but no evidence of fructi- 

 fication of P. infestans was found. The weather had been clear and warm 

 the five preceding days and the soil was much drier than on June 7. It 



