Murphy — The Sources of Infection of Potato Tubers. 359 



A practically identical result was obtained from another acre plot in the 

 same year (1917) which had similar treatment. The two last applications of 

 spray had to be omitted on half the field (four plots), while the other four 

 plots, which alternated with the first, were left untreated. There were 

 somewhat more diseased tubers in the unsprayed plots when the crop was 

 dug, but relatively much less after it was stored. The figures available give 

 the amount of disease up to November 30th only, when the total for the 

 unsprayed potatoes amounted to 12'7 per cent,, while that for the sprayed 

 potatoes reached 27'7 per cent. 



These results seemed again to show that more tuber disease might follow 

 a less severe, if more protracted, attack of blight. An examination of the 

 results does not, however, support the view that the amount of tuber rot is a 

 function of the length of the period of spore production. 



The sprayed plots in which the disease lasted longest on the foliage showed 

 the smallest amount of disease in the tubers at the time when the crop was 

 dug. On the other hand, the potatoes from these plots developed a far 

 greater percentage of the disease during the subsequent two months' storage. 

 It is possible that the disease which became evident some time after digging 

 was contracted before that operation, but that the' rot had not advanced 

 sufficiently to be visible. A dii'ect answer to this objection cannot be given, 

 because, unfortunately, the interval which elapsed between digging and the 

 advent of the bulk of the tuber disease is not known. The supposition, 

 however, is an improbable one. 



When a potato tuber becomes infected with blight, visible evidence of 

 infection under field conditions appears in about a week. That being so, all 

 the disease is visible on potatoes dug on any particular date except that 

 contracted, approximately, during the previous seven days. When, as 

 frequently happens, a serious outbreak of tuber disease follows at an interval 

 after harvesting, it presupposes, if this hypothesis be (enable, a remarkable 

 increase in the amount of infection in the week immediately previous to 

 digging — an increase which is difficult to explain. There is no evidence to 

 support the suggestion of Jones and Morse,^ that early digging favours the 

 development of the parasite in the tubers, with the result that rot follows, 

 while in the case of potatoes dug later the fungus, though present, remains 

 dormant. 



On the other hand, the great increase in the amount of tuber rot following 

 after harvesting may, perhaps, be more reasonably connected with the changed 



1 Jones, L. R., and W. J. Morse : " The Relation of Date of Digging Potatoes to the 

 Development of the Rot." Proc. 25th Ann. Meeting of Soc. for Promotion of Agricultural 

 Science, pp. 91-95. 1904. 



