MuRPHV — The Sources of Infection of Potato Tubers. 365 



The potatoes i-epoited as attacked by soft rot included those which were 

 so far decayed that the original cause could not be determined. No effort at 

 an analysis of this category was attempted because of its difficulty. Besides, 

 the amount of this type of rot in all cases but one hardly exceeds one per 

 cent., and the results are not materially altered by neglecting it altogether. 

 In the ease of plot 10, where it reaches its highest level, it is noteworthy that 

 while fifty marked blighted tubers were placed in the pit only thirty-nine 

 were recovered in a recognizable form. Some of these would have been 

 included in the soft rot column had they not been marked. It is probable 

 that the missing eleven are to be similarly accounted for. If this be true 

 for the other plots also, all the rotted potatoes should be included in the 

 blight column, in which case again no serious difference results. 



Three points are clear from this experiment. The disease which was 

 found when the pits were opened had not spread from a few initially infected 

 tubers. The pit (No. 10) in which blighted potatoes were placed showed 

 hardly more blight than did No. 6 pit, which contained similarly treated 

 potatoes without the addition of any blighted specimens. There was little 

 evidence of any disease being communicated from tuber to tuber. In most 

 cases the potatoes surrounding the marked blighted tubers were quite sound, 

 although in some cases the former were covered with decaying slime. It 

 was exceptional to find marked diseased, and unmarked potatoes which 

 had contracted disease, in contact. As the covering of the pit was not 

 absolutely water-tight and the interior was moist, the conditions were pre- 

 sumably favourable for the development of blight. 



The most dangerous source of infection is, according to the experiment, 

 direct contact of the tubers with blighted foliage. Two pits were covered 

 for some days with the blighted stalks, which were afterwards removed and 

 replaced with straw. No. 8, which was so covered, developed 13-16 per cent, 

 of disease, as compared with 3'86 per cent, in similarly treated, but differently 

 covered, potatoes in pit 1, an increase of 9o per cent. A similar difference 

 exists between pits 6 and 9. The latter, which was covered with diseased 

 stalks, showed 9'76 per cent, of blight. The former, not so covered, showed 

 O'lO per cent., the difference being 9'66 per cent, in this case. Although it 

 is well known that the leaves are the source of the spores which infect the 

 tubers, the use of stalks as a temporary covering for freshly dug potatoes is 

 still too common a practice, and, as the experiment shows, it is a most 

 dangerous one. Had there been a vigorous development of blight on the 

 leaves, the result would probably have been even more serious. 



It is clear, in the third place, that direct and deliberate contact of 

 the foliage and tubers is not necessary for the production of serious 



