MuKPHY — The Sources of Infection of Potato Tubers. 357 



should be susceptible of experimental proof, but this has not been forthcoming 

 up to the present. 



In the case of protracted blight attacks late in the season it is important 

 to distinguish between tubers in which the disease appears at, or soon after, 

 the time of digging and those which develop disease some time after having 

 been stored. Both outbreaks are often similarly explained (or, rather, are 

 not distinguished), it being held that infection is in all instances contracted 

 while the tubers are still in situ in the soil. At other times the appearance 

 of large amounts of disease some time after digging is explained by the 

 assumption that the disease spreads from tubers originally infected in the 

 field to their neighbours during storage in pits and other places. 



Evidence will be presented to show that the great bulk of the tubers 

 which develop Phytophthora rot subsequent to digging were not already 

 infected in the soil while still attached to the plant, and do not become 

 infected in storage from contact with other tubers so attacked, but that, in 

 fact, a distinction must be drawn between the ordinary course of infection 

 (which may be called " subterranean infection ") and another method, which, 

 for want of a better term, may be called " surface infection." The distinction 

 is, theoretically, not very far-reaching, but for practical purposes it is of 

 much importance. 



III.— Tuber Disease in Canada following Eaely Cessation 



OF Spkaying. 



A further opportunity occurred in 1917 of testing the conclusion arrived 

 at two years before, namely, that any circumstance which preserves the 

 foliage until late in the season but then allows it to become partially blighted 

 increases the amount of disease in the tubers. Owing to conditions over 

 which sufficient control was not possible, a succession of sprayings was con- 

 cluded nearly one month before the normal time on an experimental area of 

 one acre of potatoes. The field was divided into twelve plots, of which three 

 received no treatment, three were sprayed four times, three others three 

 times, two plots twice, and one once. It was intended that certain of the 

 plots should receive two further applications, but these were omitted. After 

 spraying operations ceased the blight became serious, but the haulms in all 

 the plots survived in part until October 3rd, when the potatoes were dug. 

 The control plots were an exception, for they were most severely blighted and 

 had been practically dead for some time. The results are given in Table I. 



