354 Scientific Proceedings, Royal Duhlin Society. 



ill which portions of plots were dug successively on September 7th, 12th, 

 loth, and October 11th. 'i'he following percentages of the crop developed 

 Phytophthora rot:— 1st digging, 21-5; 2nd digging, 33-5; 3rd digging, 13-0; 

 and 4:th digging, per cent. As to the condition of the foliage during this 

 period, it is stated that a certain amount of foliage was still green and 

 covered with spores when digging was first begun ; at the second and third 

 diggings almost all the leaves were withered, but some spores were still 

 present, while at the last date all the leaves and sialics were dried up. Eain 

 which occurred during lifting was held responsible for the large percentage 

 of rot occui'ring in the second lot of potatoes which were dug on September 

 12 th. 



While Jensen had reached the conclusion by experimental methods that 

 soil to which spores had been added remained capable of causing infection for 

 a longer period than the spores themselves continued to be viable while still 

 on the leaves, and that in fact spores which had actually fallen to the ground 

 might prove sources of infection to the tubers during the operation of digging, 

 he over-estimated the importance of infection arising directly from diseased 

 foliage, and under-estimated the part played by contaminated soil. His final 

 conclusion was that the crop was likely to decay in storage if it was dug too 

 soon, " c'est a dire pendant que les spores se trouvaient encore dans les 

 feuilles." 



To his belief in this theory is traceable the inconsistency noticeable in 

 the practical directions which he gives for the control of tuber rot after 

 digging. Thus, he found experimentally that only about 1 per cent, of the 

 spores produced on the leaves survived a dry day ; and that soil with which 

 spores had been mingled induced rot in potato slices up to five days after- 

 wards, indicating a length of spore life of five days in the soil. In spite of 

 these results it was found necessary in practice to refrain from digging for 

 as long as two weeks after the complete drying of the foliage. Nevertheless, 

 he believed that where it was not possible to delay digging sufliciently 

 long to allow an interval of even five or six days after the removal of the 

 stalks, the danger could be considerably reduced by such precautions as 

 refraining from digging during rain, beginning work at the windward side of 

 the field, digging in the afternoon rather than in the morning, and especially 

 by removing the stalks, even though digging was to follow immediately. An 

 experimental basis for the latter conclusions, which suggest an ephemeral 

 character for the spores of the fungus, and neglect the possibility of infection 

 from contaminated soil, is not given. 



Jensen's conclusions as to the danger of digging the crop while the foliage 

 is blighted were tested experimentally in the United States by Jones anc} 



