362 Scientific Proceedings, Rnijal Duhtin Society. 



It is clear from the first table that removing the stalks six days before 

 harvesting is of little avail in preventing tuber infection (compare plots 

 1 and 3). This is so in spite of the fact that removing the stalks on 

 September 24th reduced the length of the blight attack on the foliage by 

 one-third. Eemoving the stalks ten days before digging reduced tuber 

 infection to very small proportions. Here again, of course, the period during 

 which blight was present on the foliage was still further diminished. It is 

 probable that a considerable share of the success attained was due to this 

 fact. The exact influence of each of the two factors cannot be apportioned, 

 but it may be mentioned that a large amount of tuber infection resulted from 

 allowing eleven days to elapse between stalk removal and digging in the 

 experiment of 1918; and also a large amount of infection occurred in 

 experiment B of 1919, where the interval was nine days. Ten days elapsed 

 in the case under discussion, and while it is true that the weather conditions 

 (which must probably be allowed to have an influence) differed in the different 

 cases, the protection afforded to the crop is to be traced principally to the 

 initial amount of blight present and not to the interval allowed before 

 digging.i 



Turning toexperimentB, clear proof is obtainedoftheoriginofPhytophthora 

 rot in storage. The plants in plots 2, 4, and 5 retained their foliage until 

 September 18th, when they were all moderately blighted. The last-mentioned 

 plot was then dug at once, and the crop stored on the same day. The plants 

 in the second and fourth plots had all their foliage removed, and one was 

 dug nine days and the other thirty-four days later. It will be noted that 

 the period of the blight attack was the same length in all. Notwithstanding 

 this, o5'6 per cent, of the tubers from the early-dug plot had developed 

 blight by November, 21-7 per cent, from the intermediate one, and 

 8'0 per cent, from the late-dug plot. It seems impossible to escape from the 

 conclusion that during the thirty-four-day interval the parasite lost its power 

 of infecting the tubers, but tliat, on the other hand, this power persisted for at 

 least nine days after the last spore was shed from the leaves. 



As was invariably the rule in all the experiments, three in Canada and 

 one in Ireland, the greatest amount of tuber rot developed when the crop 

 was dug while the foliage was still suffering from blight. On the one hand 

 this compares with a smaller amount of disease in plots, the stalks in which 

 were removed at or about the same time as the former were dug, but the 

 crops of which were not dug until at least two weeks later ; and, on the other 



' This and other points, such as the possibility of the soil of plots from which the 

 stalks had been removed being contaminated by spores from neighbouring plots, and the 

 possibility of tuber infection during storage, are discussed more fully in a forthcoming 

 Bulletin of the Dominion of Canada Department of Agriculture, entitled "Investiga- 

 tions on Potato Diseases." 



