MiTRPHY — The Sources of Infection of Potato Tubers. 367 



(3) The amount of blight appearing in the tubers in storage, generally 

 speaking, stands in relation to the a)nount of blight present at and previous 

 to the time of digging. 



(4) The general appearance of an increased amount of disease in the crop, 

 which is not visible at the time of harvesting, but becomes so some time after- 

 wards, and which must be due to infection soon before or when the crop is 

 being lifted, can be related to no change in the conditions regularly occurring 

 before digging, but must more probably be referred to the change brought 

 about by that operation itself. 



It is not necessary to labour these points. The experiment points 

 generally in the same direction as the sei-ies carried out in Canada, and 

 proves that potatoes may become heavily infected with blight when they are 

 brought into contact or close relationship either with diseased stalks or 

 contaminated surface soil. The former possibility will be generally admitted as 

 a danger, as the result of the work of Jensen, -Jones and Morse, and the present 

 author. The view that the latter is a serious and hitherto largely unsuspected 

 source of infection is strongly supported by evidence based on laboratory 

 studies of the behaviour of the spores under varying conditions of environ- 

 ment which have been made during the past winter, an account of which will 

 be given in a separate communication. 



Summary. 



Evidence is presented that more Phytophthora tuber disease may follow 

 a less severe attack of foliage blight, occurring late in the season, tlian results 

 from a severe outbreak which runs a rapid course. 



Conditions favourable to tuber infection may be brought about in Eastern 

 Canada, or elsewhere if the circumstances are similai-, if potatoes are 

 sprayed in the early part of the season, but left untreated towards the 

 later portions. 



Under such circumstances it is important to distinguish between the 

 disease which appears in the tubers at or very soon after lifting and that 

 which appears some time later. It is the later development of the disease 

 which becomes serious following protracted or late outbreaks of blight. 



Evidence derived from field experiments in Canada and in Ireland is 

 presented to show that the bulk of the infection in the case of potatoes 

 which develop blight in storage is contracted when the tubers are being 

 dug. 



